I think people who have handlebar mounted mirrors must ride on glass-smooth roads. Even on the paved roads here I could never see much in a handlebar-mounted mirror, they vibrate like hell. On the gravel that makes up almost half my route, they're utterly useless.

Also, even when they're steady, I can't see that much. And I find it UN natural compared to driving a car - on a car I don't expect to have to look almost straight DOWN to look in my mirror - the helmet mounted mirror is directly to the left, just like in my car.