Originally Posted by KarTek

Most cars and trucks in general don't have rear sway bars or at least, they didn't used to. Without a rear bar, the vehicle will tend to understeer or "plow" when pushed hard into a corner. This is judged by the auto industry as a "safer" condition for the average driver. Adding a bar will "loosen" up the rear end. This term just means the rear will tend to slide earlier than it does now though, the car will feel tighter and corner flatter up until the point it starts to slide.

It's seems counter-intuitive but a sway bar doesn't reduce or increase traction available, (besides keeping the wheels and tires in more square contact with the ground) it just re-distributes it to other places.