I think that wearing a helmet is a smart idea. I guess that people don't wear them because were doing "dressage" and that means that our horses are supposedly under control, well trained, and there for the helmets are not needed?
Well as someone who has grown up riding horses and suffered MANY fall offs I can say from MY own experience that I can only remember falling off ONE time over a jump, all the rest of my falls have been on the flat. Also I'd like to point out that horses are ANIMALS and as such IMO they are unpredictable to a certain extent.
One of the worst falls I ever had and the only one that I ever broke anything in was a fall off a pony..... also I've been kicked in the head when a borrowed saddle busted off a horse and that gave me a concussion that affects my memory to this day..... I guess it all depends on what your head and your memories are worth to you.