That's been my concern when looking at bags. It still don't get it. If you are trying to avoid your vehicle bottoming out on the bump stops, adding bags will make your suspension bump stop a lot earlier, except the bump stop is now the bag. I'm sure its a more cushioned stop but wouldn't a wild bump on a dirt road smash you into those bags hard and early, well before the spring has done enough compression to take enough force out of it?
As I said before, the rear springs on a defender are easy to swap. If you have some chassis stands: Jack up the axle, stands under chassis, remove wheels, drop axles to full extension, remove springs, replace.
It depends how you use your vehicle day to day but you could whack in some tough springs before a big trip and return to soft springs when you get back. Not quite as fast as inflating an air bag but better for the ride, certainly offroad.
My 2 cents on Holmesy's 130. I'd say that the tray extends past the rear cross member too much for it to take much weight back there. I would be reluctant to hang anything beyond the chassis rear cross member. The amount of leverage the back of the tray would have on the chassis rails is clearly enough to apply frightening amounts of force.
I would only see the chassis failure as a Land Rover issue if it happened on an approved tub/tray. Who ever built and bolted the tray on should have established a weight limit for the tray and if you can prove that you were under that limit, they are liable for the damage. Good luck with it, hopefully your back on the road with a fresh one soon.

