This "OMG CARBON+ROCKS=FAILLZ" thing is starting to become bothersome. Rocks hitting an aluminum tube will do just as much damage as rocks hitting a carbon tube, which, to be honest isn't a lot of damage unless we're talking big rocks. If you're so concerned about downtube failure, buy a metal plate to cover it.
The supposed "benefits" of carbon over aluminum are
- Weight
- Ride Quality (Vibration Dampening, which is moot on a mountain bike)
- Stiffness, which is supposed to help with power transfer efficiency. Debatable.
- Stiffness, which is supposed to help with flex when turning.
- Bling