Similar to forward motion, vehicles should have multiple reverse speeds. These could be used for different situations where the current speed is too slow or too fast. For instance, taking fire and hitting the reverse gear to move back into defilade should be the "E speed," and while it might not be much faster than W speed, it would accelerate significantly quicker. Q speed would be likely used to back into a narrow spot, whether it is parking at a base, or just shifting backwards delicately so as to give unseen friendly infantry time to move.
One of the most useful aspects, however, would be to use to new reverse speeds as braking speeds. Currently, the only methods of braking are to let off the gas and coast, use the handbrake and risk losing control, or to press S and skid to a stop. With the new system, players can now slightly depress the brakes (Q braking) to maintain uniform speed in a convoy. Press the brakes down normally (W braking) to brake to a stop as normal--brakes will be applied slowly and evenly as with normal civilian driving. Finally, for E braking, the brakes are stomped down, and the vehicle will come to a stop as quickly as possible for emergency stops.
This system would also be useful in aircraft and boats. Certain models of boats have emergency braking systems that would take advantage of this. Aircraft would now have three air brake positions, instead of the airbrake trying to fully extend every time Z is pressed.