So I came across this article a few months ago and have been mulling it over ever since.
The short version of it is that helmet use in extreme sports (the author mostly references mountain biking and snowboarding/skiing, but the principle applies to any high risk sport) has been increasing, but so have traumatic brain injuries, the very thing they were supposed to prevent. The author points to the increase in the intensity of the sports, to the point where our safety equipment can't help us beyond a certain point. As has been pointed out, you can sustain a concussion while wearing a helmet.
Debilitating brain injuries get all the press, but the more troublesome issue is the build-up of "small" concussions over time. Get enough minor concussions and they'll have the same long term outcome as a big-time TBI. So, while helmets will most likely keep you from dying, if that's your only safety measure, you may end up with irreversible brain damage anyway.
So how does this apply to horses? Other than the fact that horseback riding is probably one of the most high risk sports available. Right now, there's a MASSIVE push to make sure everyone and their closest blood relative wears a helmet any time they're on or near a horse. And... I have no idea whether it's making a huge difference. Seriously, no one keeps tabs on these things, so while we can see an increase in helmet use at recognized dressage/eventing shows (because they're now mandatory), we have no idea what the average rider is doing. Or if helmet use would really make a huge difference. Because your average backyard/casual rider is not engaging in the same high risk behavior as an eventer or polo player. I am engaging in much more risky behavior when Royal and I canter out of the start box than when we mosey down the trail. And while humans are not the greatest risk assessors, we can tell when something is a bigger risk than another.
And this is where I think the Helmet Police approach falls short. Hyperventilating at people that if they ride a horse without a helmet, they're signing their death warrant is unhelpful at best.
Because unless they take measures to prevent accidental decoupling from the horse in the first place, the helmet is just a band-aid. If you keep riding and taking stupid risks, hoping your helmet will keep you safe, you could ending up sustaining concussion after concussion until your gray matter becomes a happy memory. The better thing to do is focus on prevention, rather than intervention. Don't get on the horse if it's scared of its own shadow. Don't ride on bad footing or places where your horse is likely to slip. And don't anything you're not comfortable with. Get off if your horse starts acting possessed. And, then, your helmet can do its job. The less you call it into action, the better it is for everyone. And the less chance you'll have of concussion buildup.
Please, wear your helmet. But also, don't do anything you'd be embarrassed to explain to the medical professionals.