I wear one. After a fall at a show not too long ago where I landed pretty much head first, smashed up the inside of my helmet and got up with a bit of a headache, I try not to ride without it.
When I have kids, they will wear a helmet until they are no longer living under my roof (if they ride!).
If someone is riding one of my horses, they don't get on without a helmet. They are riding my horse, so therefore they are my responsibility.
It other people choose to not ride in a helmet, then that's their choice. I would never belittle or berate anyone for choosing to not wear a helmet. I would expect the same courtesy in return.