When I ride, I always wear a helmet - it's automatic. I'll often wear a helmet when doing groundwork with a horse, or even when doing leading work if the horse has issues, and certainly every time I load a horse in a trailer. When I was growing up and learning to ride, I never wore a helmet, and really only started wearing one when I started jumping.
A helmet won't always save your life or prevent injury in the case of an accident involving horses, but your chances with a helmet are much better than without one. This lesson was reinforced yesterday for me when I stopped briefly at the barn to say hi to my horses, and I picked up and took home the helmet I was wearing when I had my accident last Saturday. I'm actually glad I didn't see the helmet before now, because it's a scary sight. The foam lining on the left side is compressed, there is a large crack running from the side up towards the crown on the inside - almost 4 inches long - as well as several smaller cracks, and there are holes punched in the outside - including one right through the hard plastic shell - by pieces of gravel, and a big dent on the left side to the rear.
Here are some pictures. Here is the left outside - you can see the gravel holes in the shell towards the center and at the left bottom and the light area in the top left area is the large dent:
And here's the largest crack - there are others - it runs down from the edge past the rivet towards the center:
Seeing the helmet made me realize what a good idea it is for all horsemen and women, no matter the discipline, to wear an approved helmet - it could save your life or prevent other serious injuries - it certainly did in my case. Those cracks in the helmet could have been cracks in my skull. Don't make excuses - "people don't wear them", "I don't fall off", "I'm not jumping", "I feel like an idiot", "the helmet messes up my hair", "it's hot and uncomfortable", "I'm just jumping on for a minute", "my horse is bombproof" - just wear a helmet. It could save your brain or your life.