1. Just 2 months ago, my little sister was riding on a bike with no helmet. She was struck by a car. She flew off the bike, puncturing a lung, breaking a clavicle, several bones in her face, ribs, road rash, etc. She stopped breathing at the scene. After 1 week on the ventilator, she began breathing on her own. We knew there would likely be brain damage. She woke up confused and showing most of the signs of traumatic brain injury. Fortunately just a couple months later she's done with physical therapy. Due to nerve damage she will always have a little trouble walking, but most people won't notice it. Her face didn't need surgery since the fractures healed within millimeters of their original positions. The brain is the last thing to heal, and she is a different person than she used to be, most people who didn't know her before won't notice. She can't yet hold a job or go to college. If she had been wearing a helmet it's almost certain that she "only" would have to deal with the punctured lung and a few broken bones. Her not wearing a helment not only derailed her life, but it caused major problems in the lives of everyone around her. She has 250k in medical bills and the legal minumum on liability insurance here is 25k.
2. I snowboard regularly, I've seen people without helmets die on the groomed trails. I've hit my head a few times on logs that I really wouldn't want to be running into w/o a helmet. I've never seen someone who was wearing a helmet permanently disabled by an accident.
I used to think that if you didn't want to wear a helmet it was nobody's business but your own. But when one of those cars hits you, somebody's going to pay for those hundreds of thousands in medical bills, so if you don't wear a helmet please file a DNR ahead of time.
On a "lighter" note, I tend to agree that as a society we should focus on the most dangerous things first. Right now I have a feeling that's the intersection of driving and cell phones. We can probably save the most lives by focusing on driving safety.