It comes down to money. No one is riding in the 5th when its rolling, leaf springs are cheap so that's what we get. Bags in parallel with the leaf springs might help prevent some things from getting moved around during travel but not much difference. Bags in series with the springs would help a lot but that's really expensive. And really who cares since no one is riding in the trailer. I'm not sure what someone was talking about with compressed air. Bags typically just get filled before starting a trip.