There are many advantages of a backplate and weight is just one of them. Stability is another, the combination of a solid base and limited flex of 2" webbing provide a stable platform that will not flop around like many other bcds. Further, a one piece harness is lacking all the plastic quick releases that a) break and b) release. If a plastic buckle breaks on a regular bcd it's tough if not impossible to fix. On the continuous harness there is nothing to break and nothing to "release" unexpectedly. A big benefit is the crotch strap, some traditional bcds have them but few. The crotch strap keeps things in place and allows for further adjustment of the unit. The webbing allows infinite adjustment of D rings and accessories. I could go on but I think u get the idea.
Hmmmm....
I've been using backpacks and motorcycle gear for many years that have those same kind of quick release plastic buckles. And in a lot harsher environments than floating around in the water (I.e. Rec diving). Breaking or releasing accidentally have been negligible concerns. I could definitely see how you could break one while, say, on a boat, by accidentally setting a tank down on one or something like that. But, that still doesn't seem to be a major concern. And it's not a concern for happening during a dive (i.e. not really a safety concern).
Plus, the ones I have actually looked at, if they broke, a trip to REI for a new buckle and a visit to someone with a sewing machine would have it fixed, quick and easy. Though, obviously, not ideal if it happens during a dive trip. OTOH, for most Rec divers, that would just mean renting a BC for the remainder, right? $5/day or so?
My examination of my BP/W rig leads me to suspect that the waist buckle could come undone much more easily than a typical Velcro and plastic quick release buckle arrangement on a "normal" integrated BC. I mean, it wouldn't take much at all for something to catch the end of the belt that goes through the buckle and pull it in such a way that it comes out of the keeper that holds the loose end and also flips the buckle open. And once the buckle is open, the belt and crotch strap are free to all come right apart. Maybe I just have mine rigged wrong... ?
As for stability, I can easily see how a BC could be designed to not hold the tank in a solid, stable way, but can't a good quality (design) integrated BC (which I stipulated earlier) hold a tank with just as much stability as a plastic or Kydex hybrid back plate?
As for the crotch strap, well, maybe I have that rigged wrong, too. But, from what I read, it's not supposed to be pulled up tight into the crotch. This says it should be "fairly loose" and that it should be an inch longer than what it takes to reach the waist strap:
http://dir-diver.com/en/equipment/backplate_adjustment.html
I have mine adjusted like this. I've done 6 dives with it and I've never noticed that it has pulled the slack out of the crotch strap (i.e. pulled it tight against the waist belt). So, it seems like it's kind of a safety feature, I guess, but normally never doing anything - at least for my limited use of it so far. I don't have a huge belly, but I do have enough of one that, when I did all my training in an integrated BC, I was always pretty confident that there was no way it would ride up on me. But, I could see someone with no belly, or a woman with "womanly" hips and a flat stomach having trouble with a rig sliding up if it didn't have a crotch strap. So, not that it's useless, but is it really a "big benefit"? And for everyone? Or useless for some and a big benefit for others, mostly depending on personal body build?
Anyway, please don't take any of this the wrong way. I'm not trying to argue with you. I'm just trying to sort out what things are really true advantages of a BP/W (specifically, one that weighs the same as an integrated BC) versus what things are just the "fluff" that many BP/W admirers throw out there as advantages that don't really hold up to be true advantages when considered thoroughly.
I definitely DO like how easy it is to add D rings and keepers to my BPW rig anywhere I want (though I haven't actually added any yet, but I do have some to go on when I decide what exactly I want to use them for).
OTOH, I DON'T like how much harder it is to adjust the fit of my BP/W versus the integrated BCs I have used. Not that it's that difficult, but it definitely is not AS easy an integrated BC. For example, I cannot adjust the shoulder straps without taking the whole thing off. Mine's a Deep Sea Supply - maybe others are different.
And I don't like how my shoulder straps come over my shoulders and almost immediately wrap around and under my arms. I don't have any dive lights yet, but I'm looking at them. If I get some and wanted to put a backup light on one of the shoulder straps, it would end up being just below my armpit and I'm not convinced I could get at it easily with either hand. I am seriously considering getting some of those funky 5-sided "D" rings to modify the shoulder straps with and add a sternum strap. And this didn't seem like it would be an issue on the integrated BCs I've used. Heck, the shoulder strap D rings are so far "outboard" on my body that even they are a bit difficult to reach in order for me to snap anything onto using the hand on the same side - and it doesn't matter what height I put them at. They are positioned as shown on the DIR site. My physique is just such that my shoulders slope down from my neck to the actual joint, so shoulder straps end up way out on either side. The sternum strap deal would probably help with that, too.
---------- Post added December 30th, 2014 at 12:17 PM ----------
When in the water the waist belt does little - your rig is held in place by the tension that is generated between your shoulder straps and crotch strap when you move your arms forward into the Superman flying position (as this elongates your torso, sort of).
So far, when I've dived with mine, I generally hook my thumbs into my waist belt to keep my hands out of the way. Am I supposed to keep them out in front of me?
And anyway, the waist belt (and shoulder straps) seems to be what keeps me and the rig together, whether I'm horizontal or horizontal but facing up (as when I was laying on the bottom and watching my g/f and our DM descend) or any other position. I would not think that proper adjustment would require my arms to be extended (i.e. Superman) in order for my rig to be firmly and stably mounted on my back. I would expect it to be stable and stay in place no matter what I'm doing with my arms or legs. Am I not doing something right?