I am an avid mountain biker (Santa Cruz Juliana = LOVE), roadie and commuter. I can't believe you have see a single mountain biker wearing a helmet. I don't think I have see but one true mountain biker NOT wearing a helmet.

In Anchorage Alaska, a guy was recently hit while crossing a crosswalk by a right turning car. His family says he normally ALWAYS wears a helmet, he didn't this time and it was the one time he got hit. He continues to be in a vegatative state, severe head injuries. The right turning vehicle wasn't going very fast, it is reported the driver was at a full stop before turning into the crosswalk and hitting the guy.

It only takes 1 time.

A bonk on the head can do major damage, weither it's at slow speeds, fast speeds or not. Do some internet research on head injuries and you will figure it out. Best to be protected, why take chances??