The low rear roll is not so much to gain rear traction as it is to keep too much weight from being transferred to the front. The ultimate enemy in foam tire racing is the dreaded traction roll, which can be a result of too much weight getting transferred to the outside front of the car. When you run a lower rear roll center, you are transferring more weight to the rear of the car as you enter a corner, which will help to keep it on four wheels.
It is usually best to look at the front relative to the rear before looking at left to right; as many others have stated, when it comes to car handling, weight transfer is everything.