(WHEW!)
OK, so a PVC structure will naturally be bulkier than a non-pvc structure, but it will provide long term support to help prevent collapse in the event that the concrete fails (after many years). That support also allows you to do... shall we say... less naturally structurally solid designs, provided that the PVC is capable of providing the support necessary for the rocks you're using. This means you can design your scape more fancifully if that's what you're going for.
Did I interpret that right?
If so, that explains why I wasn't terribly interested in PVC scapes. I wanted something that looked like what I would expect to see in the ocean. I wasn't necessarily going for fanciful, exotic designs. So I tended to drystack my rock in mostly sound forms, then just solidified it with the concrete so it didn't collapse if I put a pump in the wrong place, or touched it while working in the tank. I would only have run into trouble had my tank been up for years, to the point that the concrete stopped holding the rock together, and then I bumped it.
Like you, and probably most others, I did my scapes in sections. I didn't want to have to try to lift large and complex rock structures. Mike did part of his scape out of the tank, then finished it in the tank, filling it with water after the concrete had cured.