Opinions are split something like 50/50 among the people I've talked to about this. Including a lot of hikers and climbers.
I have a few base layers with high tech yarns that spread moisture out over a large area to evaporate more quickly. Now I haven't done anything scientific like measure my skin temperature with and without, so all I can really say is it's comfortable even on hot days.
Also a lot of base layers are UPF 25 to 50, so you don't have to use chemical sunblock (or reapply on a long day). I don't like the way it feels on my skin.