[Noah930]

For a more detailed discussion, you can read through Paul Frere's book The Porsche 911 Story. He tells of how the factory developed the front and rear spoilers for true aerodynamic downforce/drag reasons (as opposed to just for aesthetics). Ideally, if you have one, you should have the other.

But as Ronin mentioned, unless you're doing high speed driving (and the number I've heard tossed about is 80 mph, though I would assume that that figure's going to vary a bit, depending on the shape of the car) it's unlikely that there is much effect from the spoilers. So for most streetcars, spoilers are more cosmetic than functional. I'd be willing to bet that far more crashes occur due to poor driving technique/inattention/etc, than due to loss of control from aerodynamic imbalance at speed.