Because it doesn’t seem to matter currently if you play ranked games or casual games, the general experience tends to be the same. But one has numbers and things to go with it. You still get people playing to win in casual games and you get people dicking around having fun in ranked games, and the ranks don’t necessarily indicate how they play as a team and a whole bunch of other things that make it less than ideal.
The problem with non balanced lobbies is that they can be completely wrecked by someone who is more skilled.
This is especially true in games like Starcraft where even the tiniest difference in skill results in a total face stomp.
This is still an issue with matchmaking systems intended to keep the game balanced in developer controlled servers though, so I don’t know what the relevance is.
Its a lot less of an issue so it’s obviously relevant.