Nice try, but that is completely beside the point, since tons of people wearing seatbelts still die of head injuries in car crashes, and helmets would save lots of them. But since nobody wears helmets in cars, the idea seems ridiculous, doesn't it? Just like wearing a helmet on the trampoline sounds ridiculous. But the truth is, you stand as much chance of getting severe head trauma in a car or an a trampoline as you do skiing.
For example the pro tele skier last year who almost died of head trauma in a car accident, he probably always wore a helmet when skiing, but had he been wearing one in the car crash, he probably would have been much better off.
Again, I'm not in any way arguing against helmet use in skiing (or arguing for helmet use in cars and on trampolines), I'm just trying to give a sense of perspective.
And I've had this debate before, and it always gets everybody riled up and generally goes nowhere, so I'll stop now. Wear your helmet, be safe, have fun, peace.