I dont know why you wouldnt wear one. If you ski in a ski area, you should wear one not because of the consequences of falling, but to protect yourself from other people colliding with you. I've seen people get really fucked up from hitting other people, skis and boards connecting with the head... not good.
Out in the BC, you should probably rock one as well, especially if you're dropping stuff that has potentially dangerous consequences. I'd much rather roll over rock in a helmet than a hat, and getting swept into a tree field in an avalanche would be no good, but worse even still without a helmet.
The only time I dont wear one is when I'm goofing around on summer jumps. And I dont go for new shit doing that stuff anyways.