The rear end of this car rolls a good bit. If you run too soft of a spring the rear end will roll way more than the front end casuing the car to feel loose and unbalanced. thats why I never run anything under a 15lb spring on the rear end.
Also with the car a little stiffer it will push the tire into the racing surface rather than be abosorbed by the suspension. And as Mojo said this is a very fine line.
With a few small changes I can get my car to push or spin out on asphalt. Rear springs being one of these changes.