I concur that they're no cooler than bare arms ... when I'm training for a fall marathon in 90° and humidity in the 70+%s, I have tried just taking them off.

But they're no hotter, either. And they keep my arms from burning.

Sunblocking creams that keep me from sweating are a bad idea in hot weather. And I don't really want to be carrying and re-applying sunblock on a long run, anyway.

It's great to be able to just put a tiny little bit of sunblock on the back of my neck, my nose and cheekbones, my décolletage if my shirt is cut lower, my lips and my ears, and leave my arms free to sweat.