Well if you join a game with "Good players only please" you'd expect an even match no matter what team you're on.
In a perfect world, this would be the case. What happens in practice is people leave if they can't get on the host's team because they're worried about getting rolled by what could be a premade or worried about getting stuck with a noob who failed to read the game description (this happens a lot, but it can only be prevented by checking stats before game starts). Randomizing it would at least provide assurance that the teams aren't stacked beforehand.
The only way to deal with the noob infiltration in a "no noobs" game is to have a way to check stats in-game without alt+tabbing to bring up Pantheon, and it would be just snazzy to see that implemented as well.