It's complicated. I believe limiting queues isn't such a bad idea for now, friends could easily queue in two groups of three (at the same time, like you'd usually do) and nine times out of ten, get in the same matches (it's still a queue, you're not plucked based on MMR). Not only could you be playing with your friends, you'll also be playing against them, which in my opinion makes it far more balanced and fun than a 6 player stack. If both teams queue at the same time, chances are, you'd very likely be in the same match.
As I've said before, there's more pressing issues that should be fixed before implementing any form of MMR system.