A simple thought experiment: In your mind install your suspension helpers outboard of your leaf springs, taking some weight off of the leaves. No problem. Now put them on the inboard side, again making sure the leaves carry less weight than they would otherwise. Now move both suspension helpers little by little to the center. Once they are touching at the the center, the picture to me is very clear; extremely high sway. The moments around the center line change with the position. Yes, it's a thing, even when part of the helper's design.