But I suspect it is more necessary for snowboarders who tend to get that whiplash into head plant due to catching an edge. For skiiers, unless you are doing flips (and I don't), I can't think of many situations off a jump where the head is going to be the issue over legs, knees, and arms/wrists. (The only injury I've had is a partially torn acl. Skiing tends to be bad on the knees more than the head.)
edit
Also, to be honest if you are on green and blue slopes, a helmet is overkill unless you are one of those maniacs that scream down the hill and have no idea how to stop.