The robots that use continuous rotation servos are small and light enough so the double ball bearings inside the servo are enough. For larger robots, the servos are too slow and weak so DC motors are used. In this case, a shaft like in the picture you attached may be used, especially in Batle Bots. People use chain and sprockets to transfer the motion from the motor to the shaft when they need to lower the speed (and increase the power) or they are using shaft couplers to connect the motor shaft to the wheel shaft. If you need to eliminate backlash, you may use toothed belt and cogs instead of chain and sprockets. But you have to take into consideration that they are reducing the efficiency.