all the studies I've seen indicate a reduction in serious injuries of around 50-80%.
That's interesting since all the studies I've seen show most injuries happen to the upper extemities, followed by the lower, followed by the torso and then comes the head/neck (as a combined area, but you can separate them to reduce their %).
Most common injury is to soft tissue. The most common serious injury is a broken bone.
The studies you're looking at are likely a reduction to serious "head" injury.
In (let's see...1968...2013) 45 years of riding and working with, well, a LOT of horses
(I'd never remember every horse I've ridden), I've hit the ground (gates and other objectss) more times than I care to think about but never (knock on wood) hit my head on anything...yet... although I've had a couple of shoulder injuries, cracked ribs, sprains, etc... Know of far more cases of broken bones and soft tissue injuries beyond remembering. Have a cousin who was launched into a tree when the horse went down while running. Broke her shoulder and give soft tissue injuries to her face (shoving her braces into her lips). She wasn't wearing a helmet (none of us did back then....it was a working farm), but it wouldn't wouldn't have made a difference if she had been. In fact I've only known (personally) to people who suffered serious head/neck injuries. One (who was not wearing a helmet) was hospitalized with a serious concussion, but luckily she survived it. The other was wearing a helmet, but a broken neck killed her (her head was ok though).
I agree it is pretty much playing the odds, but it doesn't provide any protection for the majority of injuries that are sustained. Even so would never discourage anyone from wearing one and would encourage them to. I make my grandchildren wear a helmet when they ride and I encourage my children and girlfriend to, but only because it makes me feel better (which they feel gives them license to lecture me for not wearing one
.....especially my girlfriend). However, even when they don't wear a helmet I'm more worry about neck and back than I am their head.
Of course they make padded vests and even neck protection too now days. That would reduce the risk to mainly the extremeties which still make up over half of the injuries reported.
Riding horses is dangerous. As a % more people are injured each year from horses than from motorcycles or cars (you ladies make up the largest number, but what do you expect with more women riding now). With or without helmets we ride at our own risk and take our chances or we give up riding and find something else to do.