1. RE: Weight... It's my bet that a wet AL backplate and harness weighs less than a wet TPII.
2. Exposure protection: I don't understand why anyone would want to dive without at least something on... In springs and other freshwater climes, the thermal protection is great. Unless you're diving in water near 100*, even a 3 mil isn't too much. Plus, it offers some protection against scrapes and bruises from encrustations, rock, rocking boat decks and the like... In saltwater, it prevents stings from jellyfish, which are common even in tropical climes.
3. Assuming that you insist on diving with no protection, imagine for a moment whether holding 40+ pounds of metal cylinder and first stage directly along your spine (with padding) is more comfortable than having a wide, flat plate distribute the weight across the entire surface of your back. Yes, there's some truth to the fact that some people like the padding over no padding... But even those that swear by it admit that underwater, when everything is weightless, the padding is annoying. And frankly, topside, I personally see it as ineffective at keeping that cylinder off of my vertebrae.
No, the wide, flat plate that distributes the weight over the muscle in my back is much more comfortable to me.