Wearing short sleeved merino by itself?

Merino is an anytime material . It’s core property is that the fibres are very fine and structurally more complex compared to a more general wool product.

You can wear it on a hot day to feel cooler as the fibres will absorb sweat and keep it next to your skin or on a cool day to keep you warmer as it’s very insulating due to a hollowness in the fibre.

It doesn’t gain as much of a body stench compared to an artificial fibre and can be worn multiple days in a row without being too offensive to you or other people.

When a 100% Merino shirt has worn out you can dump it in the compost or use it to line a plant pot.

You’re right in that people use it as a base layer as it works well next to the skin and on hot days you will get more UV protection.

It is more expensive compared to other materials but the benefits are high for the investment cost. I haven’t worn anything else for cycling or any outdoor exercise for a long time.

I have a light weight merino wool top. Sometimes I wear it as a base layer, and sometimes by itself. I find that it breathes really well, and I know that wool does not retain moisture well.

The nice thing about a merino wool layer, is that if you get one and you don't like riding in it, it is nice for lots of other things.

I think the most important thing here is personal preference. There ain't no right or wrong.