This depends on how the crash plays out. I have absolutely completely rolled, though as you point out, that usually does not happen.

But whether or not you actually roll, the idea is the same -- you want to dissipate as much energy from the crash into sliding and rotation with impact distributed over as much of strong areas of your body as possible. You want to avoid direct impact with point areas as that maximizes damage to you.

This stuff is not hokum and if you watch the motorcycle vids that wphamilton posted, you'll see those guys are good at falling. But one difference between falling with a lightweight bike that you're mechanically attached to and falling very heavy motorcycle that you're not attached to is that with the bike you're better off staying fully attached rather than getting partway disconnected and opening up other opportunities for injuries.

I learned to fall when studying martial arts a zillion years ago and found the training incredibly useful in cycling, skiing, and kayak crashes. You rarely have time to think during crashes, but if your kinesthetic sense is trained or naturally helps you do the right thing, it makes an enormous difference.