I don't really have trouble with mandatory helmet laws so long as they only apply to minors. Motorcycle Mandatory helmet laws I'm sorta on the hedge about. In the end, people should allow to do activities that are risky only to themselves.
(Two asides: this doesn't address the efficacy issue, but if helmets prevent injuries, then it is reasonable by the above argument to mandate their use. Secondly, it is arguable that motorcycle riders who don't wear helmets provide more to society through organ donation than they take in life-long medical treatment.)
Motorcycle riders already pay higher registration fees due to the higher likelihood of injury.