If your truck sags in the back then the load is too great for the factory springs. Adding more leaf springs, either to the current leaf pack or by adding SuperSprings preserves the ride and handling of the truck as it was originally engineered by the manufacturer.
With air bags if one fails then you are in trouble. Air bags do not provide the same dynamic dampening and load support that leaf springs provide. Air bags need to be adjusted depending upon changes in the load. None of these problems with SuperSprings. A installed a pair on my truck in less than one hour and that was it.
SuperSprings only come into play when the load is too great for the factory springs. The 1-ton pickups use the same approach with a second leaf pack mounted above the first and the second set only is engaged when the first set is overloaded.
After I added the Supersprings the truck drove better with less sway in tight turns and the shocks performed better as there was a narrower range of movement.