I'm a bit of a hypocrite... I believe everyone should wear a helmet, but I often get on my boy without. I ride him in from the pasture in a rope halter and lead, bareback, with no helmet. I have an excellent emergency dismount, though, and if I feel threatened I get off. I also ride helmetless at the beach, if I intend not to ride along the sand (he is quite strong if I ride along the sand), because I don't like my helmets getting wet and salt water can damage the foam.
HOWEVER, I have had a very nasty fall, about a year ago, where I galloped my boy, then tried to pull him up and he wouldn't. In the process of doing whatever it took to get the result I wanted, I lost both my stirrups, and my balance, and had to give up on trying to pull him up if I wanted to stay on. There wasn't any room to circle him, with thick trees on one side and a deep ditch on the other, so I had to just let him run himself out. I couldn't get my stirrups back, and found myself sliding to one side. Eventually I was clinging desperately to the side of my horse, looking for a soft place to bail out. I lost my grip before I could find such a place.
I don't remember hitting the ground. I do remember getting up and walking after my horse, and I distinctly remember realising my arm was hurting horribly and thinking that I'd broken it (I had fractured my humerus, a hairline fracture just below my shoulder joint)... I don't remember getting back to the road, or getting into any cars. Next thing I remember, I was sitting in the front seat of an unfamiliar car, next to a guy I didn't know (idk why I remember he had short dark hair), thinking that if I wasn't so out of it I would tell him to stop and let me out. Don't recall getting to the hospital, or any of the time I was there. My pupils were different sizes for a week, and my helmet had a HUGE crack from front to back and scrapes with dirt embedded in them so badly I couldn't get it out. That could have been my head :/