Are tubeless tyres an upgrade from tubed tyres, or just different?

Yes you can patch larger holes that the sealant can't seal on its own using "bacon strips" and a reaming tool (linked video is a Muc-off one, but there are loads of different brands of bacon strips out there). It's basically a rubberised plug which gets inserted folded using the reaming tool, which then allows the sealant to do its job.

On the odd occasion that you can't seal a hole with a bacon strip and need to use a boot (I've never had it happen to me), you should still be able to apply a boot if you empty the sealant out, give the tyre a clean, apply the boot and use a tube (always worth carrying a spare tube, just in case).

Every time I've had a puncture with tubeless tyres on it's sealed itself very quickly, often before I've stopped moving. Then it's just been a case of pumping a bit of air back in and away you go.

Another advantage in addition to self-sealing for most punctures, is that you can run much lower pressures without any danger of pinch-flats - something which is really handy if you're riding gravel or trails. Also no pinch flats if you hit a kerb or pothole wrong, especially with a fully loaded bike.

I ride tubeless on my adventure tourer / gravel bike and tubed on my road bike - next time the road bike needs a wheel upgrade I'll go tubeless on that too.

Whether you personally should care about them is up to you, and depends on how heavily loaded you ride, what sort of surfaces you ride on, how often you get punctures and whether it's something you're open to, really.

What if the puncture is not adequately sealed by the sealant? Can I patch a tubeless tyre in a permanent way with a glue-type patch? A permanent repair without a slow leak for inner tubes is easiest when you always carry one spare tube so that you let the glue dry before using the newly patched tube. Unless some major form of technology breakthrough has occurred of which I'm not aware, if a tubeless tyre can be permanently repaired at all (which I suspect won't be the case because of the sealant "slime"), you should always carry one spare tubeless tire with you to let the patch glue dry. Carrying a spare tire takes more space and weight than carrying a spare tube.

No, if the sealant and specific plugs (mentioned in Diado's answer, the same ones that are used for automobiles and motorbikes) fail to seal the hole, but the hole is not extreme, you can just swallow your pride, insert your spare tube, and carry on riding. If the hole is large, you will need a tyre boot in the exactly same way you would need it for a normal inner tube tyre. Then at home, after a good clean, you can indeed use a tube patch or one of the larger tyre-specific one. If the hole required a boot, it may also require some sewing. and then covering by a polyurethane glue and a patch.

What if the cords of the tire are damaged? With a tube-type tire, there is no sealant "slime" so I am certain that an adhesive tire boot will stick to the inside of the tire. However, with tubeless tires, the sealant might make it more difficult to ensure the adhesive tire boot stays in the intended place.

That is unlikely. Quite the opposite, it will make it stick. Or at least be neutral.

So, why should I care about tubeless tires? I see them as a non-repairable solution that harms the use of emergency tire boots and at the same time encourages the rider to cause rim damage.

It is repairable (I repaired several punctures myself) way that makes better riding properties especially off-road (lower pressures) and reduces the number of punctures you have to deal with (no pinched tubes, small thorns are plugged). Small unsealable holes can be plugged without taking the tyre or indeed the wheel off the bike.

Any potential rim damage is the responsibility of each rider, their selection of the tyre pressure and their choice of their riding style.

The user error requires the presence of three conditions: (1) the tire must have a narrow cross section

Pinch flats can also happen with MTBs.

(2) the tire must be underinflated

Low pressures are selected for good riding properties and it is impossible to perform well in certain off-road events (cyclocross, cross-country) with such high pressures an inner tube would require. The other option is a tubular, but that is even more hassle.

(3) the rider must go over a bump at too high speed.

Stones and roots may be hidden by mud, leaves or appear suddenly behind a corner on a trail. Sometimes they cannot be avoided even on a maintained course, less so in unmaintained trails.

• Mountain biking is specifically riding over terrain full of "sharp objects".
• MTB tires often die because of sidewall damage or because the tread blocks have worn down so much they've lost their "bite", which happens way before the tire carcass wears down to the fabric plies.
• As for pinch flats, tubeless tires give you the ability to safely pinch flat. I know that sounds weird. You can ride with underinflated, low-volume tires and hit obstacles without worrying about patching a pinch flat every time. You don't need to sacrifice comfort and rolling resistance anymore just because you need to protect against pinch flats. Conditions that would pinch flat a normal tubed setup are within the normal operating envelope for a tubeless setup.
• Rim damage is not a big problem for MTB. Rims are disposable wear items. For example, my personal "best pressures" when it comes to smoothness and grip result in my rim bottoming out on large impacts. I find this risk acceptable for racing and other "important" rides. If you pinch flat with a tube, you're likely to have damaged the rim too because the rim has to have hit the ground in order to cut the tube. In order to completely protect the rim and tube, I'd have to run far higher pressures, decreasing grip and smoothness. You pretty much have to pump your tires rock solid with tubes: a single pinch flat event is fatal for your tube and you'll be forced to pull over and patch it.
• Sealant tech is pretty good. If you do get a large rip or tear, the aforementioned bacon strips are an option, as well as simply throwing a tube in there. You can still use tubes with a tubeless tire. If you ride in conditions where sidewall tears are expected, you can get "gravity-oriented" tires with protected sidewalls. As a personal anecdote, I once picked up a large nail (~5mm diameter) on my way to the local trails. I didn't even realize it was there until I got home and saw this massive metal rod sticking out of my tire; there was virtually no air loss before the puncture sealed. Let's see how well tubes do there.

To address your final two points, sidewall tears for a tubeless tire are just as repairable as those on regular tubed tires, and rim damage is an acceptable sacrifice to make in search of higher performance.

I'll cover some of the potential advantages by discipline, since the original question didn't state a specific discipline or disciplines. I'll address both advantages and disadvantages.

Off tarmac: MTB, gravel

In these scenarios, the advantages of tubeless seem very well accepted. You are more likely to encounter puncture and pinch flat hazards on short notice, perhaps before it's possible to react. Tubeless sealant will seal most small punctures in this scenario. Also, tubeless dramatically lowers the risk of pinch flats - note that it's still possible to pinch flat some tubeless setups, it's just that here, the tire itself is getting pinch flatted, rather than the tube.

The issues you raised are not really issues. If you damaged a tubeless tire enough to need a boot, you'd have done the same to a tubed tire. If a hole is too big for the sealant but too small for a boot, you'd use tubeless plugs for tubeless tires, whereas you'd likely have had to change the tube on a standard clincher.

Performance Road

The advantages of tubeless over the alternatives don't seem as clear here. I know one shop mechanic who was a tubeless enthusiast, including on the road. I have talked in person and on forums to a number of riders who say that they had tried or helped set up tubeless tires, and that they still preferred tubed clinchers. I have not personally tried road tubeless.

Jan Heine wrote in 2017 that he did not recommend tubeless pressures over 60 PSI, which he felt might cause blow-offs. If this happened during a ride, this would cause a loss of control and a crash. Heine noted that in almost all other applications, tubeless tires are run at low pressures, typically less than 45 PSI. He seems to feel that performance road tire presures may be in an unsafe zone for tubeless tires.

(Interesting side note: Zipp recently released two models of its 303 wheels, the 303S with an internal width of 23mm, and the Firecrest with a 25mm internal width. On both wheels, they state a maximum pressure of only 72.5 PSI, and their recommended pressures for most riders are very low. At my weight of 133 lbs, my recommended pressure for a 25mm tire on the 303S is 58 PSI front and 60 rear, whereas I typically run 75-80 PSI with the same width tire on a 19.6mm internal width rim depending on the surface.)

Tubeless may have been touted as having lower rolling resistance than clinchers. (NB: the tire and tube lose energy through hysteresis. Bumps in the road compress the tire/tube system. As the system rebounds, some energy is lost. No tube = less material to rebound after compression.) However, for high-performance road tires, where tubeless and standard clincher models of the same tire exist, Aerocoach documented that tubeless clinchers had similar rolling resistance to the standard clincher versions with latex tubes. (NB: latex has much lower hysteresis than butyl.) They also stated that rolling resistance on a tubeless compatible tire set up tubeless and the same tire set up with a latex tube was comparable. Bicycle Rolling Resistance also documented this with MTB tires, and they also documented this with the Schwalbe One tubeless and the One V-Guard tubed tire (comparing latex, light butyl, and normal butyl tubes). Of course, a tubeless road tire should have slightly lower rolling resistance than a comparable road tire with a butyl tube.

Josh Poertner, owner of Silca and former employee at Zipp, discusses latex tubes on a podcast here. They are a bit trickier to mount and they do require daily inflation. If you twist the inner tube while mounting, it will likely blow out, and this phenomenon may have given them a poor reputation for durability. This is user error. Poertner reports that latex tubes are actually more resistant to punctures and pinch flats than butyl tubes. I personally use latex tubes on the road.

Note that as the standards for tires and rims solidify, the tire-rim fit is likely to improve, thus improving the reliability of tubeless tires further (especially in terms of consistently fitting to rims). Their rolling resistance could decrease further with evolution in tire materials and construction, although I'm not sure how this would improve their position relative to clinchers with latex tubes.

One unanswered question is how reliably road tubeless seals after a puncture compared to lower-pressure tires. My hunch based on feedback from some users I talked to was that road tires might lose too much air before they sealed - remember that performance road cyclists use much higher pressure than gravel cyclists. I wasn't able to substantiate this, however.

Cyclocross

Current tubeless tires are probably not optimal versus the traditional gold standard of tubular tires. However, the logistics for tubular tires are very different from tubeless tires, and most amateurs and many shops may lack the skill to glue tubulars. Either setup should be superior to tubed clinchers, based on experience in other non-tarmac disciplines.

For some time in the early 2010s, CX tubeless tires had a reputation for burping in corners, i.e. the seal broke just a bit and you lost air. This is obviously bad. In cyclocross, riders often run tires at very low pressures, e.g. <20 PSI, to increase traction while cornering. Tubular tires have been regarded as the optimal setup, provided they were glued very tightly to the rims, because they can be safely run at these pressures. They are also very hard to pinch flat, and they obviously can't burp in the same way that tubeless tires can. I haven't surveyed professional CX racers, but I suspect many of them might still regard tubulars as optimal.

Moreover, UCI-regulated CX is limited to 33mm tires by regulation. This stems from the evolution of the sport, but I believe it persists only due to tradition and regulation. Amateur riders, even if they participate in a UCI-regulated race, will not be disqualified for using wider tires than this. In any case, it may be that the optimal CX pressures and narrow tires make it hard to eliminate burping in corners.

Alternatively, as tire to rim fit continues to improve, this issue might also disappear. Even in 2016, November Bicycles reported that with their preferred rims and tires at the time, they and their riders experienced very little burping. Cyclocross Magazine reported similar in 2018, and they noted that tubeless tires were getting some uptake by professional CX racers.

In any case, amateur riders can simply show up with their existing gravel wheels and presumably tubeless tires. I don't perceive that the advantage of tubulars is enough to justify getting a separate set of wheels and either learning to do a proper glue job or paying someone to do it. (NB: if you don't glue well enough, you can roll a tubular tire off the rim when cornering. If you crash and you take someone down because of your sloppy glue job, you need to apologize to everyone profusely and pay a professional.) Another very minor point is that on the road, tubulars seem to have higher rolling resistance than the best clinchers. In part, this might be attributable to the layer of glue. If true, this would also apply to CX tubulars.

Touring and commuting on road

Riders here benefit from wide tires. Wide tires mean lower pressure than performance road settings. As stated above,I expect tubeless tires' benefits relative to standard clinchers to improve with wider tire width and lower pressure. So, perhaps there's another possible use case here, especially since the sealant should defeat many small punctures. Last, I'd expect many of these riders to strongly not prefer latex tubes, so tubeless tires should be the lower rolling resistance option also. (NB: you could split the difference with light butyl tubes.)

I am not familiar with tires used in this space, so I can't really flesh this section out. It's my sense that like many things in cycling, tubeless tires originate from the high-performance side of the sport, so manufacturers may simply not have thought to develop good touring and commuting tubeless tires. Here, I'd expect the net benefits to tubeless to exceed the performance road scenario. The question is, does that justify a switch to tubeless? Some commenters on other answers said that they had benefitted from the sealant, so your riding conditions may be the main determinant here.

Tubeless tires are objectively better in many aspects. Whether you care about those advantages or not is a different question.

If you ride off-road or even on bad roads, you might want to reduce your tire pressure to provide for a smoother and more efficient ride, or for improved traction on loose surfaces. In such circumstances, a lower risk of pinch flats is a valued upgrade.

For MTB tires tubeless tires have significantly less rolling resistance. On road tires it's a mixed bag: with the Schwalbe One tubless was about the same as a butyl tube, but on the other hand all the lowest rolling resistance road tires are tubeless. You may not care about rolling resistance. Or you may not mind latex tubes or tubular tires, which are other ways to get low rolling resistance.

If you ride in a city where there's broken glass in the bike lane, or a climate with plants evolved specifically to puncture bike tires, you will appreciate the additional puncture protection provided by tubeless sealant. Before tubeless existed, my city bike had puncture resistant tires, Slime filled tubes, and a Mr. Tuffy. All this extra "armor" easily adds 1 kg to the bike compared to a tubeless setup which can get away with a light and supple tire and still be immune to small punctures. And even with all that armor I still got flats, so I switched to airless tires which were even heavier. But if you don't mind heavy wheels or repairing flats, or your riding conditions are free of debris, this may not matter to you.

You can of course get tires which are lighter than a complete tubeless setup, but puncture resistance will be much worse, and probably unacceptable for any city / offroad / goathead climate scenario. And you'll also be using lightweight or latex tubes, and reinflating your tires pretty frequently.

Tubeless tires can be repaired, except you won't ever have to repair a small puncture. Bigger punctures can be repaired with tire plugs which don't even require unmounting the tire. If you're having a really bad day you can always throw in a tube with a dollar bill until the tire can be permanently repaired. It just takes a little careful cleaning of the sealant to get a permanent repair.

On the negative side, tubeless setups require a little more maintenance. The sealant might need to be topped off as much as every 4 months, and probably yearly you'll want to unmount the tire, clean out any dried sealant, and remount the tire. Rim tape installation is more critical, as it must be air-tight. Mounting the tire can be a pain if you don't have an air compressor: some tire & rim combinations pop right on with a floor pump, others not so much. When you do have to unmount the tire it's a bit of a mess.

In the MTB world, nearly everyone runs tubeless these days. On the road, things are slower to change, and there are options (tubulars, latex tubes) that can compete with or beat tubeless setups if puncture resistance isn't required. I personally run tubeless on my road bike also, because I ride it in cities where I'll encounter broken glass and goatheads, and to get comparable rolling resistance and weight I'd have to go with a latex tube or tubulars, neither of which I find very appealing.

In addition to everything that has already been said, don't forget that another serious advantage of tubeless tires is the weight. Loosing tubes will save you about 100 grams of each wheel and wheels are the first thing to take care of when lightening your bike. Moving parts have Moment of inertia. The further away from the axle is the weight, the more resistance you will encounter upon acceleration. Heavy wheels can be compared to heavy shoes of a runner, he will inevitably get exhausted sooner than the lighter counterpart.

I believe they are an upgrade for the following reasons:

• I live in a reasonably civilised country with well-maintained roads but I get several flats a year usually. The trouble is that if a sharp object is tiny, the same thing that makes it impossible to see in time makes it more likely to puncture the tyre due to the increased pressure.

• I've seen (very) old tubes where the patches had degraded and I've also come across patches slowly developing a leakage channel from the original puncture site to the edge of the patch, even though the patch was originally correctly applied. Because of these and other problems many home repair guides and bicycle repairmen recommend changing the tube after a handful to a dozen patches, depending on whom you ask. Tyres don't return to a new state and after a while, for one reason or another, they will become practically unsalvageable.

• As for the suggestion that the lack of a possibility of pinch tyres ‘encourages’ rim damage... Both are actually discouraged by the extremely uncomfortable and also heavy ride caused by underinflated tyres. There's nothing encouraging you to damage your rims, and certainly not by driving across speed bumps. Those are uncomfortable enough as it is, even with fully inflated tyres.