This has been suggested many times and I don't think it would change. You also can leave the game what I do if teammates doing nothing or if players misplacing towers. And I don't really get why 2 players would be perfect or maybe 3. The maps can therefore be better adapted for 2 or 3 players each team. Maybe the problem is misplacing tower, I already made a suggestion about it but it wouldn't help that much, this has been suggested many times they don't much about it because it remains a good game for the players. I don't think they want a change, maybe new maps, but that's ok.
Back to the 2v2 3v3 part. I agree with you, you can carry each other, but you also can play with six players. Not every team is bad. I think the most of the people know what they're doing. I mean, it's just building towers on the right place and sending monsters on the right way. I think if you play it more, you can be better at it. I think six players is good enough but maybe the towers have to be set at the right place automatically so it would be fair and it won't need a change for 2v2 mode of 3v3 mode.