I agree. I first started out doing trail riding when I was about 6 or 7, my parents didn't know much about horses and I was riding with a family member who doesn't wear helmets so no one thought anything of it. After I started taking lessons and getting into more serious riding when I was 10, I can't think of a time I haven't worn my helmet.
Even if you have a trust worthy horse a helmet very well could save your life.
The BO where we board now ex-husband was riding a 24yr old mare who was as quiet and as unspooky as could be. The BO and her daughter came back from the grocery store to find him laying on the ground and the horse just standing there. The man now has the mind of a 7yr old and lives in a nursing home. A helmet very well could of saved him that. These weren't inexperienced horse people, they've been all over the country showing.
I read another article of a little girl who was killed in a situation that could of been avoided. Her and her mom, who was very horse-smart went out on a quick evening trail ride, the horse spooked and the girl came off. The horse stepped on the girls head and busted it open.
Even just a couple weeks ago, a horse and rider were hit by a car in the town over. The girl's brains ended up scrambled on the sidewalk.
I'm not saying a helmet is going to protect you from everything, because it's not, but they can prevent a lot.