When you prop up the rear with helper springs or air bags it doesn't transfer ANY addition weight to the front axle which is the problem with those fixes. A properly sized (sized for 15% of the trailers GVWR) and properoly adjusted WD system will allow the maximum rated tongue weight to be carried but will NEVER have any effect on the trucks GVWR or AXLE ratings except that they will both be reduced by the weight of the hitch itself. It is absolutely the BEST way to distribute the load and maximize your towing capability.
Good luck / Skip