I'm not convinced the case for helmets in skiing is as good as that for biking but that's a personal opinion based largely on anecdotal evidence.
I mainly do off piste and touring. I wear one for steeper off piste lines, lines threatened by ice/rock fall and those with jaggy rocks sticking out.
I just don't get this. I think I go faster on my board than I can on my bike. There are lots of things covered in snow but there are also a hell of a lot of hard things that aren't. Rocks, trees, people, lifts, hidden drops, misread speed and bad choices!