I would honestly like a duo queue in casual CC so I can take my friends in and teach them. There's also no stake in casual matches and the skill level disparity is so wide that the reason someone lost a match would most likely not be entirely on facing a premade either.

However, ranked CC should absolutely stay solo. A well-coordinated WHM+DRG LB can instantly wipe out the entire enemy team with no counterplay. A 2 person team that is coordinating can swing the match way too far in their favour because of the small team size, and that's not acceptable when you can lose ranking from it.