Too many accidents, too many injuries. Over the course of my life (I rode BMX for a while when I was a kid), I've broken a few fingers, a few ribs, a collarbone, dislocated both shoulders, put my head through a car window or two, and have had terrible road rash. Oh, and I've broken a couple helmets too. The stupidest one was trying to whip-skid in SPD's while drunk in the rain. There are very few things in that sentence that aren't stupid.