As I've said before, it all depends what we are talking about here. Road or off-road, or is this discussion being general and ignoring the actual use? Given the nature of this forum I would have thought there'd be more of an off-road bias, but maybe not.
Personally I'm of the opinion that on-road and off-road helmet benefits are entirely different. I have little interest in on-road cycling myself anyway, but I would assume that the actual number of incidents are relatively very low compared to off-road and likewise the benefits of a helmet in a road incident is perhaps lower.
Off-road is a very different situation, although it partly depends what kind of riding you do (saying that I've seen serious incidents on a tame trail as much as on serious trails). Personally I think you're mad to go doing things like downhill stuff without a helmet, and ideally full face.
I don't insist anyone has to wear one, it's up to you, but just about everyone I know wears a helmet for what they believe is a very good reason. Many as a result of having an accident or their wearing is reinforced by an accident they've had or witnessed
What I will say is if a particular location is private and demands a fee or membership to ride, and if they say helmets are required to ride there, then you wear a helmet. End of.
I have serious doubts that those who are fairly anti-helmet have ever actually had an accident or witnessed someone who has had one. I would seriously like to know the experience and opinions of those who have had one (mainly off-road) and whether they still believe in helmets. Personally I believe front line experience from those involved counts far greater than cold academic studies based on sources that aren't fairly comparable or accurate, or the studies are somewhat generalised. Just saying that a helmet has a certain percentage effect in reducing the risk of a head injury, doesn't paint the entire picture about the benefits or otherwise of a helmet itself. The way I look at it is you are very likely to sustain a head injury in a crash, helmet or not. That is not what I look at out of a helmet. I'm more concerned about whether I will survive and/or avoid ending up paralysed. If I get a head injury that is recoverable from and live another day, then the helmet is doing the job. In my opinion, and based on personal experience.