Heres why I almost always wear a helmet. Its not because I do dangerous shit and crazy high risk lines (well, part of the reason, maybe), but because of other people.
I dont trust anyone skiing around me. When I fall, I usually have some amount of control, and can try to minimize the risk to myself. I can also be really careful in dangerous areas. But one thing I cant plan on is some dumbass screaming down the hill into my back when I'm lazily skiing down a groomer.
Yes, rocks, trees, rails and decking out on an icy pipe can fuck you up bad. But a snowboard edge to the face or back of the head sounds a lot worse. Thus, I wear a helmet because of other people. And even if the resort was empty, I'd still wear one, just because of the dangerous shit I do. Its not that I dont have confidence in my ability, its just that I know I'll fall and nail my head sooner or later.
But thats just me, and I'm sure someone out there has a great reason not to wear one.