Mark Prior doesn't do the towel drill because he got hurt, he's done it since at least USC, if not longer, to keep his arm loose and to keep his form, if I'm not mistaken.. But funny, original joke anyways. The towel drill is just a checkpoint to make sure everything is correct to be able to smack the glove from max stride plus 5 feet. It does nothing as far as improving mechanics or endurance, it just tells you that if you're not hitting the glove, something is wrong. If you miss to the left or right it is a balance issue and if you're short, you're rotating too soon. Of course, you're always better off having someone there who knows what they're doing and can breakdown your delivery at each important stage. I've never understood this odd fascination with the towel drill. Someone needs to get over excited about pitchers doing long toss, at least that improves arm strength.