I do, but I also ride under 90 minutes, and usually under 60. If I spent all day riding in the Arizona sun, I would wear a cowboy hat instead.
I also figure adults can choose what level of risk they are willing to take. Some reject all riding as too risky. Others reject some riding as too risky. Some wear helmets, and some do not. I'm not their nanny or daddy, so it is their business. I loathe people on both sides who ridicule or demean the other side
. If safety was paramount, we wouldn't even groom horses - there are some studies indicating the risk of injury is greater handling a horse than riding on the flats. I've come as close to getting my head busted dealing with a scared horse from the ground as riding one.
I'm not sure how a helmet protects against being kicked in the face. FWIW, the main reason I wear a helmet is that my mare is a klutz and I could see her falling with me on a paved road or rocky trail. My body would probably survive a sideways fall, but I doubt my head would survive a sideways smack on pavement or a boulder, regardless of how often my wife tells me I'm hard-headed.
In honesty, if I owned a more reliable or agile horse, I might not wear a helmet. Yes, accidents can always happen, but safety involves analyzing and taking precautions based on probabilities, not possibilities. Adults should be aware of the risks, and then allowed to take whatever steps they think are needed.