Weight is not neccassarily a bad thing if you are runing in the dirt. You need some weight to keep your tires hooked up and moving the vehicle around fast enough to keep up. You can put a motor with wheels on a stick and make it go, but without enough weight it will just spin the tires out of control and go nowhere. You can strip a truck down and make it half the weigt of other trucks, but if you cannot get the power of the motor to the ground what good is it. If you had a life size muscle car like a Camaro, Mustang, or a Charger and you wanted to make it go faster, would you strip it down just to lose weight and make it go faster? Hell no, you would put the biggest power plant you can find in it so that when you mash the throttle it will strip the asfault right off of the road.
Of course you want to make sure that you are not hording un needed weight for no reason, but you also need to make sure that you keep the structural integrity in tact so it can handle the rigers of runing in the dirt. Your real challenge is to put enough power in your vehicle to make it go as fast as you want, and still have the weight to get the power to the ground so that power is put to good use. You also need to find the right driveline components to hold up to that power so you are not replacing parts after every run.
The other view of this is of course you can run any vehicle as lite as you wish when you have angelina jolee to tend to your wounds weather you win or lose.
My Revo is right around 10lbs including batteries. Like Jerry2Kone said, there is a point where a heavier vehicle helps traction. Too light=no traction. Too heavy=just carrying extra weight for nothing.