I am biased I guess. My father is a neurologist and I have heard of so many accidents where a bicyclist, motorcyclist, skier, etc. wasn't wearing a helmet and is either dead or sustained serious head trauma that could have otherwise been prevented.

Also, when I was about fifteen I was cruising down a hill on my bike. I hit a patch of sand and gravel from a house that was being built and flew off my bike, with my head slamming right into the corner of a brick mailbox. My helmet look like someone hit it with an axe. I walked away, and had a lot of appreciation for my helmet.

Another one of my friends was riding and hit a bad patch on a sidewalk and went over his handlebars and woke up in an ambulance. Again, his helmet saved his life.

I guess you could also make the argument that if you knew how to drive you shouldn't wear seat belts, because knowing how to drive is what matters most. Accidents happen and wearing a helmet is a little bit of insurance against head injuries. I can't think of a compelling reason to NOT wear a helmet.