Actually this, or vortice generators would be a good aerodynamic addition to our car. Anyone who has seen the way snow or water get blown off the back, you'll notice that there's a delamination of airflow over the center of the back window and a low-pressure zone at the bottom of that dead area.
Adding aerodynamic devices to the top of the car would either get rid of that low pressure zone, decreasing drag and increasing fuel mileage, or increase the amount of air hitting the rear wing, increasing high-speed downforce/parasitic drag.