I haven't a problem with people wearing them though it seems a bit silly when people think they are 'muscle' tops. Stood about not really doing much.
If you don't like getting sweaty and want to wear one by all means do. I personally always train in a vest as I find the sleeves on t-shirts get wedged on my upper arm and I spend too much time pulling them down. Likewise the bottom of the shirt rides up my middle when doing pullups/chin. I've 4 black vests and I only ever train in these, I get so sweaty during CV I can and have had to ring them out into a sink.
I've got a Nike Dry Fit top I brought as it was going cheap though I've never actually used it. It feels great to wear and obviously unrestricted movement though I'd just feel like a bit of a tool working out in it personally.
Like people I see wearing hats and all sorts of head gear. It's not cold and it's not sunny so it's unnecesary. Sorry to anyone who trains in a hat
