it's meant to keep them from slapping/vibrating, stockers are like you describe but the top one stays flat when driving on pavement so that's why there's no need to have them clamped. The stock one don't have that much arch in them either. It also helps with axle wrap quite a bit to have them clamped together. I doubt you'll gain any travel as your shackle will be straight before the need of lowering the leafspring more, maybe 1'' but at that point you won't have weight or barely any on that wheel.
edit: it would possibly make them softer on small bumps in the road, but that all depends on what leafs you have on and what arch the shorter ones have in them.