With air bags you're also running the risk of bending the chassis at the contact point of the bag.
Unless you add similar reinforcement to the contact point for the air bags, you're introducing a weak point. I would suggest that you don't become another statistic with a bent chassis (which could = vehicle write-off).
My d22 has a canopy , heavy duty steel roof rack, 2 spare wheels on top of the roof rack, drawers in the tub and a two tonne trailer everyday, air bags pumped to 50psi, Air bags have been in for 2 years now and have not bent the chassis. D22s are tougher than your average ute!
Your still creating a shear point on that part of the chassis, its like turning the chassis rails into a see saw with alot of weight on each end, after a while the rails just fail under the strain due to having a load put on them that they are not designed for. Using heavy duty leaf springs has the added benefit of creating a double shear point on, as said, two much stronger parts of the chassis. I saw a d40 fail like this on fraser island, not the cars fault just a retarded owner...
Now I am not saying you can't have airbags but just like anything in life common sense has to be applied and you can't assume just cause you can pump the bags up more to take the load that the chassis can deal with it as well.