My mom was out riding with some of her friends. They were going about, just a walking gaiting ride. As usual, she wore no helmet. Going down the hill, a group of horses came running up on them from behind, then kept going. All of the horses in the group got very worked up. My mom's horse, Bo, is extremely competitive. There was no holding him back. Her friends watched as he put his nose down to the ground and started running down the hill after them. My mom went quite a distance, trying desperately to rip her horses head up, or do a one rein stop. His neck was locked. Finally, going downhill around a corner, she flew off. Bo had sense enough to stop.
Her friends finally caught up. My mom had broke her left collarbone, wrist, and leg. The worst injury, her ear was ripped all the way down to her lobe, hanging off just near her earring. Her friends got her into the car, loaded the horses, and took her to the ER. She completely lost her short term memory due to a concussion. Her ear was saved, and her bones healed. From that day on, my mother always wears a helmet.
It shouldn't take someone going through an ordeal like that to get them to wear a helmet though. While it is a personal choice, I recommend everyone does. Sure, I look silly, and my head gets a little sweaty, but I feel that's worth the protection.
Wearing a helmet has never caused me to have less fun on my horse, but I'm sure a major brain injury would.