Winter. It gets cold here (was 11 degrees last night) I get cold no matter what I wear for cloths. I have wear a hat to keep warm, that kinda defeats the purpose (at least for me) of shaving. When I ride the harley up to the mountains I have to use sunblock, but I don't see that as much of a downside thought, if I have hair I have to put sunscreen on my face anyway, so just spreading a little around the top is no big deal.
I have not hit my head on anything yet so not a downside for me, at least not yet.
I have a friend who rides, he had curtains, once he seen me shave my head a while back ago, he shaved his, he said I inspired him (he only knew me with hair). He is sly now, his wife likes it better too. He looks younger with no gray curtains.