Heh... might not be a bad idea. Yah, kinda tough to shoot while mounted but maybe a look and point grenade launcher could buy a little time to find cover? Or network with an overhead UAV?
Though in Afghanistan there's also something to be said for just getting some horses and mules. I believe the Special Forces have done some of that already.
I'm sure better military minds than me have thought about this before, and since they don't use dirt bikes, there probably is a very good reason. But it would be very nice to be in the 53rd Dirt Bike Cav Unit. (Motto: Wheelies of Death).