i dont wear one, but i probably should.
mainly to prevent serious concussions. my friend was one of the best hockey goalies in the province. then a kid on his team, who had a shot equivelent to some nhl players shot at him in warmup when he wasnt looking. it hit him in the jaw and gave him a huge concussion, couldnt leave his house for like 2 weeks. he can still do all the stuff he could, but he's never been the same and he ended up quitting hockey.
the moral of the story is that concussions are caused a lot by slamming your teeth and can happen really easily when you're skiing.