I have personal anecdata that wearing a helmet made the outcome of an accident worse. I don't wear one. I don't go around telling people they shouldn't wear one, in a ZOMG YOU ARE SO GOING TO DIE YOU IDIOT kind of way. I do get narked when people say I should wear one because they are beneficial, or suggest that any injury sustained while not wearing one is my own damn fault and if I end up a vegetable because I get hit by a truck well, I had it coming.
Here's my opinion:
Are helmets beneficial? Maybe, sometimes, in limited circumstances, for limited definitions of "beneficial".
Can helmets make things worse? Yes, definitely, in limited circumstances.
Is it guaranteed that I will hit my head in such a way that a helmet would have helped should I fall off? Absolutely not.
Is the chance of falling off and hitting my head in such a way that a helmet might help greater than the combined chance of falling off and either not hitting my head at all or hitting my head in a manner that means the helmet makes things worse? No. Not even close.
So there you go. What you put on your head is up to you. I have made an informed decision and anyone who tries to tell me I am an idiot, or that I've made the wrong decision, based on flawed assumptions and an inadequate risk assessment, can shove their plastic casquette somewhere damp and moist.
This isn't, for those who are slightly disabled in the comprehension department, about being anti-helmet. It's about being anti-compulsion. It is perfectly possible to be pro-helmet and anti-compulsion. Those of us who choose not to wear helmets have no problem with whatever anyone else puts or doesn't put on his or her head. The problem arises when people argue (incorrectly) that they are 100% beneficial, because that's the start of the slippery slope to compulsion.
Sam