I personally think is just depends if your racing or training or commuting. If your training or commuting the discs are nice because they stop well even if the rims are wet and covered with snow. On the other hand if you're racing you want to save as much weight as possible, especially when you're lifting that thing up over your shoulders a few dozen times which is why most goes with cantis. The only huge downside to discs that I can think of is that they may be too powerful, have you ever hit your brakes hard on your road bike, it goes all over the place since the tires and all are so skinny, I'm assuming the same thing would happen with discs on a road/cross bike. I could be wrong however, that just makes sense to me.