I've worn a helmet about 75% over the last 5-6 years and I think it's a good thing in general, but the funny thing is I've only had one skiing inury that a regular helmet would have helped. That was a fall in the bumps where I got spun around and landed backward and smacked my head and ended up with a concussion. (stars, memory loss, the works.)
I've also broken my nose and busted up my lip real good and a helmet wouldn't have done much for either of these injuries.
If you use a snowmobile alot to acces the backcountry I would definetely suggest a full face. I have a full face for slednecking and it's saved me a few times.
If you don't mind some weird looks in the lift line and the heat, I would do it if you think it's gonna help.
On a side note, I can't believe how may people ski with sunglasses vs. goggles. I never ski hard with sunglasses because my goggle have saved my eyes and nose from too many ski tips to the face.