Re: strengthen pvc against shore rocks etc
Your shoe analogy doesn't correlate well to the long-gone OP's posit. If I were walking in a factory with nails on the floor - I would much prefer to get a shoe with a sole that had a higher degree of resistance to penetration vs something that didn't. That, or not walk on the floor or get the nails off the floor.
If I were dragging inflatables onto a shoreline covered with nails, sharp glass shards, or newly-formed obsidian I would much prefer to have an extra layer on the surface of the pontoon most exposed to these materials. While PVC, Hypalon etc are incredibly tough I can guarantee you that if I used a ice pick I could poke a hole faster through one layer than two.
At the end of the day - inflatables are not all that fast so the extra hydrodynamic drag of reinforcement probably isn't a huge factor, nor would I be taking a inflatable into those areas where repeated bashing and cutting are everyday events. In Alaska, aluminum craft are norms. Especially for landing craft, or boats which need to be beached every so often.
If we were to get crazy about actual performance vs. opinion we could always ask for objective information from the Gary King company, or actual users of reinforced bottom inflatables. Note post #4.