Mountain biking can be a very dangerous sport if you aren't skilled in it. That's why I think it should be mandatory to wear helmets if you are biking down public trails. Save yourself the headache.
I had a friend who never wore a helmet. He was a mountain biker and a road cyclist. For some reason he thought an accident would never happen. He couldn't have been further from the truth. His front tire hit a rock one day and got got a concussion. He blacked out for hours in the hospital. Since that day he always wears a helmet. I'm so glad he does.