Just to add my two cents... This is a subject my friends and I have talked about several times while patiently waiting for our premade (GASP) custom game to get off the ground. What we've found is good players will rarely stick around after looking up our records. Players that hate premades will flame us, then leave (at least this provides us a good laugh). We're left with people that are so new to the game they don't know what a premade is, and players that just don't care.
Why does this happen? Why don't we mix up teams? Because stats do matter. People look them up and judge you by them. _I_ look them up and judge others by them. This may be fair/unfair, but it's how things work. My friends and I have no desire to screw up each other's stats.
Why don't we play other teams? The system, as stands, discourages this. Not only this, how often do you see 2 premade 5v5s at once? Or even 4v4 ones. We can't find each other, and even if we could, it's too much of a problem to try and join the other premade match with out being able to talk to each other. By the time we co-ordinate who's game we're supposed to be joining, that game already has people in it.
Team auto-match in skirmish seems to be a good answer to this. (There may be issues with say... a 4 or 5 man team finding another 4 or 5 man team to play against, but that's a seperate issue entirely.) I also wouldn't mind an option (emphasis on the word option) to play custom matches as unranked. Just an additional ranked/unranked button in the already existing lobby options menu would be sufficient. I can't imagine that would be hard to code, but I'm not a coder so who knows. Sometimes the simplest thing can be the worst.
-- Tukulis