(We should probably make a new thread if we're going to talk about helmets, but I'll give my two cents while it's fresh in my mind.)
I, personally, always wear a helmet. Last summer the one time I took it off, my horse and I almost galloped off of a 20 ft cliff into a cement ditch. ("We're going for a trail ride! Western saddles are super safe! Let's start galloping! Oh crap, my horse doesn't steer anymore and we're veering off the path toward that drop off! Wow! If I hadn't violently yanked my horse's face back on the path we would have died!" etc) I've also fallen on my head twice coming off of a horse. The first time, I smacked my head so hard I was seeing pink flashes and couldn't remember locations for an hour and jogged for a quarter mile on a broken leg (I was so out of it I felt no pain) trying to get back to the barn, (horse took off with me and I did a very failed emergency dismount) and the second time my horse randomly bucked me off during a quiet hack around the barn. I hit the back of my head hard on the ground, and actually suffered from whip lash in my neck and back. Both falls are documented in dents in my old helmet.
I'd like to maintain my motor skills, thanks very much. Yes, it's true that my computer could blow up right this second and kill me, or that my rabbit could suddenly turn monty python on my butt and eat me, but I demonstrate extreme caution when working around animals because they're really unpredictable. I even make a point to putting on my helmet before picking out my horse's feet. You never know if right when you bend over next to that hock if a horse fly might decide to have a snack on your pony's tummy...