In my many years of adult biking, I've seen a several accidents where the biker went over the bars, in two of those the helmets were destroyed, basically split in half from the impact. I can only control some elements of risk, but it's pretty foolish not to take care of what you can.
I'm not very fond of rules and generally only follows the ones that I think are sensible. That means I occasionally speed, don't always come to a complete halt before turning right at red lights, and tear the labels off mattresses. :)