I'm an XC rider myself, so maybe my interpretation of the term "dirt jumping" is incorrect, but the guys I was referring to generally jump off these man-made dirt "jumps" (for lack of a better term) that are about three or four feet high. These jumps usually shoot them pretty high in the air (I'd guess at least ten feet, maybe more) where they do some crazy tricks and then land on the downslope of another jump. It's basically like motocross with mountain bikes. That's dirt jumping, isn't it? (I'm not trying to be a smart ass, I'm really not sure. The most air my bike sees is when I lift it down from its hook in the garage).

Anyway, I was hanging around watching them last summer and I noticed that they all ran v-brakes on the back. I assumed, like you, that disc brakes would be preferrable. When I asked them about it, they said that when they're coming down on a jump and need to stop immediately, the resulting torque created by the stopping power of a disc-brake versus their mass and velocity can snap the left chainstay or seatstay. I can't see why they'd make that up, because most of their rigs had disc mounts and they were all riding pretty nice bikes with discs on the front, so money obviously wasn't a concern.