Since trombone seems lost in a statistical thicket of contradictory data and inconclusive evidence in which it is impossible to prove beyond doubt that helmets are a benefit to everyone at all times, I'll simply refer him to Pascal's Wager.

Simplified, Pascal finds it rational to believe in God and salvation - even without evidence - rather than not believe, because it costs him nothing to do so, he risks little, and he may ensure his eternal rest in a blissful heaven. If it turns out there is no God and there is no heaven, Pascal has wasted nothing.

In the search for salvation, belief in God is simply a safer bet than disbelief.

For "God", now substitute "wearing a helmet."

http://en.wikipedia.org/wiki/Pascal's_Wager