There's a lot more structure in the rear door jam/body corner than a door, not that a door couldn't be beefed up but that too will make the door heavier putting (again) more on that hinge pin wear point.. My spare has got to weigh 85lbs, add the tire carrier plus beef up structure and I'd be a 110lbs easy, those hinges will only tolerate so much of that. On the other side with the toolbox and generator I'll be in the 120lbs range.the argument for a separate swing out is often that the doors won't support the weight of the swing out and load. They would start sagging, implying deformation of either the hinges, the hinge jambs or the door itself.
Doesn't mounting the swing out hinges to the door hinges put the same load on the door jambs?
Thinking along the KISS line, why wouldn't some strategic backing on the inside of the door be sufficient to hold a rack mounted on the outside of the door?