Well, from a performance standpoint, you don't need downforce at the back of a front wheel drive car. As I mentioned in a post a few months back, however, I once had my friendly Pontiac dealer (who did seem to otherwise know what he was talking about) tell me that the rear "wings" raised the effective level of the tail of the car, smoothing the airflow out as it passed over the roof and off the back of the car, and helping lower overall drag. He did note that the fuel savings from the better aerodynamic efficiency might or might not be canceled out the added weight (the wing) that the car had to carry. So at best it's a wash, and it probably hurts performance because of the weight. Some people want to take them off, but they're probably the same people who remove their valve stem caps to save unsprung weight....
