Using a 10 speed Shimano 105, do the shifters ever wear out? Should I get a spare just in case?

Many shifters can are used for the whole lifetime of a bike. They are also pretty expensive.

It is also very likely that there wilm be replacement 10-speed shifters available in future, but it is true that Shimano switched to a different cable pull.

Anyway, given the price of the shifter and the lower likelihood of needing to replace it, I would NOT buy a spare just in case. If you were really unlucky, you might be forced to buy also a new rear derailleur in the future. But at that time, if the shifter is worn out, the rear derailleur might well be as well and a new RD is cheaper than an unused shifter in the cupboard.

Shifters are rarely the weakest point in bicycle maintenance. Their operation does not deal with big loads nor aggressive environment, compared to what chain, chainrings, tires etc. are subject to.

Shifters can be damaged in a crash, but everything else can as well. They can also become filled with gunk, but there are cleaning tricks to flush it out. If shifters' internals break, it is most likely because of a manufacturing defect, not wearing forces.

Benefits of having a spare pair of shifters are few:

• In an unlikely event of your current controls break, you can quickly replace them.

But disadvantages are many. Expected lifetime of current shifters is 50+ years, and during all this time:

• You'll have to waste storage space to store the replacements.
• You can loose the spare when moving, it can be stolen, you can forget where you put it, it can be damaged while laying around.
• Your whole bike can become damaged/stolen, making repairing just the shifters unreasonable.
• You might upgrade to a better/different bike, selling/giving away the current one.
• You can still break shifters while far away from home, and be forced to buy a replacement in a local shop, rendering those at home useless.

In the end, I do not think it is worth for planning that far.