It's a complicated issue as their are pros and cons and they vary with environment.
Ideally a wing is for lift, a dry suit is for warmth and a lift bag is for lifting things or shooting a line to the surface, and life is better when those functions don't over lap.
It's also nice to be able to say that your configuration should be "balanced" and that you'd be able to swim it to the surface in the event of a wing failure when wearing a wet suit. However, in my experience the circumstnaces where that is possible are severely limited. A diver in a 3mm wet suit diving double AL 80s to 150' in Open water can probably manage it. A diver in a 7mm wet suit diving steel tanks with an AL 80 stage in a 250'of water facing a much more challneging swim. (And divers who think they can swim up 25 pounds from 250' need to seriously rethink and re-assess their abilities uinder real world conditions.) Finally, the same 7mm wet suit diver in a 200' deep near vertical section of cave that is then still 1500' from the entrance has still different issues and challenges.
Practically speaking I have encountered instructors who insist on a double bladder wing, or two stacked wings if I am wearing a wet suit. The idea is clearly that a drysuit will allow me to over inflate the suit to develop the required lift in the event of a wing failure, and thus in no dry suit is present, a redundant wing is required.
The dry suit as redundant buoyancy approach works, but in open water it may or may not be my first choice. Absent a dry suit, I would also consider shooting a bag or using some types of bags as a rudimentary BC by clipping it to the scooter ring and dumping gas as I ascend. But more to the point there are some downsides to dry suits in some environments and in my opinion diving dry is sometimes not worth the trade offs, especially when the rationale is to plan for the dry suit as redundancy for a very uncommon failure.
In that regard the downsides of a dual bladder wing (if it is properly designed and does not add excess bulk or complication) can be less in the big picture, than diving dry just for the sake of redundant buoyancy.
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Let's consider a just completed cave diving trip. AL 80s and an AL 80 stage was the norm, the depths were 90' or less (usually averaging around 40-50'), the water temps were around 72 degrees F, and the dive times were in the 2.0 to 2.5 hour range with one dive per day. I dove a 5mm wet suit and a dual bladder Nomad. In this case, I would have been comfortable with a single bladder wing and no redundant buoyancy as the rig would in fact be "balanced" and any weight on board was ditchable - it just wasn't worth swapping wings. However had the caves been deeper and steeper or had steel tanks been in use, then a double bladder wing would have made sense with a wet suit. My team mate also dove wet, and a large part of our decision was simply weight, bulk and air travel baggage limitations for the trip.
Another team on the trip dove dry, in part because they store much of their gear at the destination so weight/bulk/air travel baggage issues are not an issue. However during the week there were several dry suit related issues relative to leaky seals, failed seals and leaking/fizzing inflators, as well as some equipm,ent related issues that may have been rleated to storing the equipment at the destination as opposed to using it on a regular basis. It's an approach that obviosuly adds some new factors and complications.
In any case, both teams had successful dives and lots of fun, but one of them worked a lot harder at it between dives.
However, I also noted that my team mate, who normally dives dry, while I normally dive wet in water 70 degrees or warmer, experienced a significant improvement in SAC rate. I realized I had over time let my SAC rate slip and increase to match hers. Once I adjusted back down to my normal SAC on the next dive, we then both had several hundred more psi to use on most dives for penetration than would have been the case in dry suits due to greater streamlining, and we noted generally faster swim speeds at a normal level of effort. Over the course of the week we ended up with more bottom time and saw more cave than would have been the case in dry suits.
In short, it's nice to have different tools in the tool box and it's a good idea to not get stuck in any kind of dogma that then prevents you from using the most appropriate and optimum tools for the specific environment.
