Personally, I always wear a helmet, and I insist that my son do so too. The way I ride and the places I ride, I've ending up hitting the deck about once a year or every other year, crashes where wearing a helmet has helped. I also get very wary when I see other people not wearing helmets in traffic, as it signals to me that this is someone who doesn't cycle much and is happy to take chances. This is because in Norway helmets are pretty much the norm.
However, I've read enough statistics on how helmets won't help in serious crashes, and the negative sides of mandatory helmet use, to realize that my view probably is heavily tinged by habit. I probably could ride safely without a helmet, and I'm sure other people could too. In Denmark they do, all the time. So I try not to be too black and white about it.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett