If I can, I wear a helmet. Anyone who rides besidesmy mom or I (friends, family, etc..) wears a helmet. We have about 5, one being mine. In my area wearing a helmet is almost unheard of, I have only seen one other person wear one. My friends and I have actually gotten made fun of for wearing helmets, but I prefer my head in one piece/functional.
I am very prone to head injuries, and had a pretty serious accident unrelated to horses (14 days in ICU) when I was about ten, and managed to get by with frecuent mild headaches. Then, when I was 12 I fell off of my previous horse and had gotten a concussion. It wasn't bad, but it was enough that I couldn't remember about 2 days. I've had some other falls since then, but no injuries besides being sore at the most.
One story that always made me wear my helmet as often as I can was a trainer that was about 10 minutes away from our previous barn. She had been riding her fantastically trained dressage horse, who had an off day and reared and ended up falling on her. She had a head injury very similar to mine when I was ten, only not as bad as mine(I assume because I was younger I healed better). She ended up, for a lack of a better term, brain dead. I am not sure what happened to her but last I heard she had passed away due to complications of her accident.
So, to me, it's like buckling your seat belt. Sure, you don't have to, but it can really save you in tough situations if you do (another thing I always do lol)