Suspension Air Bags

When I would haul my ATV in the bed of the pickup, then attach the TM, it was way too much weight for the WDH and I couldn't level the rig. So I bought some of the Firestone Aig Bags for the rear suspension. They work great. I am now able to level the truck easily. So, what I am wondering is when i tow JUST the TM, why can't I use just the bags to level the truck and no WDH? Has anyone tried this? Seems like it accomplishes the same thing.