The only problem is that when you fall (if you fall) you bleed more :evili don't know about streching, but listen to this. this forensic cop comes into the dealer i work at. he says that if you drink pedialite, and pop a couple asprin that is better then streching. the asprin thins the blood sent we are all bent up in the riding possition, and the pedialight is what gives you the nutrients.
As far as i would think, the only time you would actually have to thin the blood is if you have bad circulation or a bad heart in which case you should not be riding anyway :lolthat would be a lot of aspirin! ^^^
I know this from my karate days, we didn't stretch because it doesn't do anything. A stretch will relieve pain for a few seconds, maybe a minute or two if you stretch really really hard. But in general, stretching before you ride, lift, or do karate is useless.