Me and the guys I wheel with run some of the toughest trails in the southeast and we have ran about every type of leaf spring setup possible. The rigs with shorter, lower lift springs consistantly performed much better in our area. The trails we run are generally slick rock ledges and creekbeds that offer very little traction and the lower COG rigs hook up much better on the slick rocks. The long travel springs tended to unload and axlewrap very badly, just because a spring has more flex doesn't mean that it's transfering more energy to your rig.

I think all things equal a soft rig might have more weight transfer than a stiff rig, with the 60/40 weight bias our trucks have this will mean that on a steep incline the weight might be 40/60 for a soft rig and 45/55 for a stiff rig, which might suffice to explain the traction difference ?
Same therory as the buggy axle winches ?