Moreover, while I wouldn't suggest NOT wearing a ski helmet, I don't see any compelling reason why one should -- and I say that having looked into the issue. Head injuries are quite common in cycling, but rare in skiing. Moreover, in those cases when a head injury while skiing does result in a fatality or serious injury (in recent years, the deaths of Sonny Bono in 1997, and Michael Kennedy a few weeks later come to mind) were the result of direct, head-on impact with a tree. No helmet in the world is going to save you when that happens. Moreover, especially in children, there is some evidence that a helmet increases the risk of injury because it can cause the head to whip back and forth during a fall, damaging the neck and spine.
I always wear a helmet when cycling, and often when climbing. I don't wear one yet when skiing, and ultimately may never do so. But that's a decision everyone should make on their own.