Its all to do with grip balance, a part of Stoners style. Its to do with being able to adjust the lean angle of the bike to moderate grip rapidly and when needed. Being able to flex or extend the arms is much faster than adjusting the hole body to affect the lean of the bike.
Heading into a corner, lean forward and in, then pull the bike inwards towards you and the rear grip gets looser. If its too loose you can rapidly pick the bike to bring the back in or pull the bike more into the lean to get it more put again.
How far forward or back adjusts front rear grip balance also, ie. if you want to drift out, eg. to pass somebody in a corner, body goes back a bit and the front tend to loosen up a bit and drifts out with out as much speed loss. But mainly with Stoner he likes the front planted so we see him way up forward ....... I'm a wuss like that too
Its all about having a counterweight that you can move around rapidly to affect grip. Its also safer to let go if the bike high sides cos it usually just gets ripped out of you arms or you can moderate the release of the bike better and not get catapulted so bad.
There probably is a minor effect from wind drag however this is not the reason as the effect varies way to greatly depending on wind direction ie. say you have a headwind of 50km/h then the variance can be 100km/h depending on what corner you are at !!! crazy!. Don't know why this guy didn't realise that.
There really is a great benefit to grow up riding dirt.
Here's a bit of a start:
http://www.youtube.com/watch?v=P6d7j-sWFn4&feature=endscreen
