At polocrosse on the weekend, a really good friend of ours came off her horse. How? NOT galloping around after the ball, turning on top of other horses, pushing other horses flat out... She was practicing her ball skills at a WALK, her horse tripped, and fell on topof her. She was drifiting in and out of concioussness for about ten minutes, had three ambulances present, and was rushed to hospital. Thankfully, she is ok.
However, I do not doubt that if she didn't have a helmet on, she would have been DEAD.