I wear one for jumping lessons outside an arena (in my mind for some reason there's more reason to? - don't ask me to be logical), cross country because it's dangerous and they ake me anyway, and for riding horses I don't trust, especially over jumps.
I hate the thing. It's hot and uncomfortable and makes my riding worse. It's EVIL. But I wear it anyway. *grumbles* You'd never see me wearing it for normal 'safer' stuff though - I try to get away with not wearing it as much as possible. Which maybe I shouldn't do, but hey ho.