When I grew up, we rode in the back of pickup trucks and on the top of haywagons. Now we know better and wouldn't consider doing it. Children wear helmets to ride BICYCLES and not HORSES? That's pretty dumb, in my opinion. No matter how 'dead broke' your horse is, it has a mind of it's own. I rode horses without helmets in my youth, I would never consider doing it now.
A friend of mine was on a trailride with her niece. Nice calm, easygoing trailride. They crossed a paved road. No traffic, no rain, no nothing. For some unknown reason, the horse stumbled hard, she came off, hit her head and DIED on impact.
Another friend of mine came off, hit her head and went into severe convulsions ... she doesn't remember any of it. Was in a coma for most of the day, but fortunately has healed with no ill effects.
No, it won't prevent all injuries, just some of the most severe, potentially life-threatening ones.