[Al Skierkiewicz]

You would severely limit the current available for the motor controller by using the DSC as a power common. Not only that, the DSC would then be forced to generate noise as the DSC common varies with motor usage. Remember there is no "ground" on the robot. There is only power common which is something altogether different. As all motor current returns to the battery through normal (two wire branch circuits) wiring to the PD, noise is not a problem for the DSC, Crio or the radio in most cases.
Also for all the newbies lurking here, the robot frame may not be used as a power pathway. You must provide two wires to all loads connected to the battery and they must be insulated from the frame.