Well it is possible though to be aware that you're not wearing a helmet, and not do things like "speed through the corner"

I am 100% helmet on mtb trail rides though. My fall rate there is high enough that anything else would be stupid. When mountain biking, I don't just wear a helmet, I very often USE a helmet. Just a few weeks ago I slid out sideways and in the fall whacked my helmet pretty good. Gave it a dent, and it was like 4 years old and getting really gross anyways, so I bought a new helmet and tossed it. My new helmet (that I didn't wear on Monday) still feels fresh and clean every time I put it on.