I'm not a DIR guy and I don't dive in a wetsuit, so maybe I haven't really thought about this as much as I should. But my reading of a "balanced rig" is that you can (1) be neutrally buoyant at your shallow stop without much gas in your tank(s), and (2) swim your rig up from depth without ditching weight or using your BC.
I don't think that there is a really good argument for ever ditching weights at depth - I suppose that if you had a very thick wetsuit and the combination of lost buoyancy from suit compression and the weight of gas in full tanks added up to more than you could swim up, then you would need to. But you would be better off not diving in that configuration. Ideally, if you had to bring a large amount of gas and dive in a thick wetsuit, you should be able to swim up enough to get some suit expansion, and make it to the surface without your wing (with an SMB as a redundant buoyancy).
The total amount of ballast that you need could be ditchable, non-ditchable, or some combination, but it doesn't change that analysis. The advantage of ditchable weight in my opinion would be for an injured or tired diver at the surface if they couldn't easily maintain positive buoyancy with their wing, or in a rescue situation on the surface.
The
GUE website has this to say about double bladder wings: "Double wings are an invitation to a disaster - do not use them.". They do not elaborate further, and I haven't really heard a good reason why this is so. Maybe one of the GUE/DIR people can tell me. Again, I don't like diving heavy rigs in a wetsuit, so maybe it's better not to need a double wing, but if you do need redundant buoyancy, it seems to be as good an option as any.