Interesting brochure on the OEF group page tallking about helmets and rider injury statistics.
http://www.horse.on.ca/safety.html
I work with a coach and a clinician, both of whom are Olympic competitors in dressage (one is a medal winner) and both ride with helmets every time they get on the horse. With them setting the example, virtually all of their students ride with helmets as well. It's easier to get a helemt on your head than dressage boots on your feet, so what is stopping people from wearing helmets? Are they embarrassed? Afraid that it will seem uncool?