As an EMT and as a long-time cyclist , I've seen more bike related head injuries than I'd like to think about. The ones that really stick with me (because they are people I know or work with) are how serious some of the slow/fallover injuries can be. I really believe that most of the 8 or so "slow" type head injuries that I remember could have been prevented or at least the damge reduced by wearing a helmet. I think Dnvrfox makes a good point about some accidents that a helmet won't help, but I wouldn't ride without one.