Why I wear one:
Other People. Helmets are nice when you fall. They keep your head from bouncing and impacting on ice. But the main reason I wear one is protection from other people. While I'm in control, many other people that ski are not. Thats why I wear one in bounds. Yeah, they're nice when you crash, but when I fall, I usually end up hitting my hip or knee or twisting my arm somehow. A helmet wont really save you if you hit a tree glade skiing anyways, but it certianly will help you when a 200 lb skier crashes into you from behind.
Out of bounds, I wear my helmet anyways. Dropping cliffs and skiing out in avalanche country is dangerous, theres a lot that could happen out there. Helmets are really a must when I'm out of bounds or in BC.
The only time I dont wear one is when I'm sessioning a kicker somewhere, where I know there is no avalanche danger and out of control skiers.
So, why would you not wear one? Helmets these days are lightweight, warm and daresay even stylie. Besides, even if you only use it once, you'll be glad you were wearing it. Wear a helmet, its that simple.