OK. It's clear there is little chance of progress here, because it just seems obvious to people that helmets work. But the more one looks at it, the less obvious it becomes. The central question that must be answered is, if helmets are so effective at preventing injuries, how is it that there seems to be no real-world correlation between increased use of helmets and reductions in serious injury? In Australia, for example, making helmets compulsory has not resulted in a lower rate of injury to cyclists.
We can debate that here if anyone wants, but there's a perfectly good thread on the subject in A&S. It's about ninety pages long, which might deter the casual reader. But for the benefit of those who think the issue is cut-and-dried,
here's a link to a post in that thread made by a BF member who is a doctor treating patients with neurological injuries. As you'll see, he doesn't recommend helmets on the grounds that while they prevent abrasions etc, they may actually increase the chances of the type of injury that causes brain damage.
I'm not qualified to judge, but I have read widely enough on the subject to know that he is far from alone in that opinion.