Mountain biking is one of the situations where I'd actually recommend wearing a helmet: the surface you're riding on tends to be loose or slippery, with the result that you're more likely to simply fall off. There are also things sticking out of the ground to bash your head on, as evidenced by the dent in your helmet. None of these apply on the road, however, where you're far more likely to get flattened by a bus than to simply fall over, and there don't tend to be jagged rocks sticking out of the road.