I had a really bad mountain biking wreck about 12 years ago. Since then, I've been religious about wearing a helmet.
Basically, I was heading back to the campsite hauling down a dirt road. Now, I've ridden that road God knows how many times. Well, I come ripping around a blind curve and there's a gate closed! The gate has always been there, but it had never been closed in all the years I had ridden there. I slammed on both brakes and was going to try to lay it down, but I had no time. I slammed into that gate going about mach II. All I remember is the sound of the impact and being airborn. When I woke up it was dark and two people I didn't recognize were standing over me. The next thing I remember is being back at the campsite. I don't remember anything else until the next day. I found out that no one knew who those guys were, they just happened to come by and I guess I was able to incoherently mumble enough information to them that they were able to get me back to my campsite. Inspecting the damage the next day, the bike was destroyed. The entire frame was warped. My helmet (yes, I was wearing one) was cracked almost completely in half with a rock embeddeed about 1/2" deep in it. The only thing that saved my teeth was that I bit my toungue. I bet completely through my tongue and still chipped three teeth.
There is little doubt in my mind that if I wasn't wearing a helmet that I wouldn't be typing this.