I'm maybe missing something, but I don't really see the harm in allowing a maximum of a duo to queue into casual CC provided they also balance it on the other team?

If it's just a maximum of 2 and they always queue against another duo, 2+1+1+1 vs 2+1+1+1 seems fair enough, and if there's no other duo, the duo queued will stay in queue until another duo they can match against is found.

I don't see any potential harm in such a system for casual queue because the reason you lose in casual queue is so varied that it cannot be pinpointed on one factor alone, it could range anywhere from simply being outplayed in a fair match to losing because of that 0/8 MNK with 0 healing done.

Sure, 2 people coordinating is good enough to wipe the other team out in some cases, but the other team would also have a duo that can do the same thing.