I'm gonnna have to disagree with the poster and the commenters...
Premade teams are NOT against the rules, not unethical, and exactly what custom games are for.
Unfortunately they are also very frustrating to play against as a pickup team for 2 reasons:
1) in a pickup team you can have one or more very unreliable players who don't want to coordinate or communicate.
2) no matter what you say, there is a communication gap when you don't know your teammates you don't have when you do. Chatting (until they have in-game voice) is just not good enough.
These two things DO give an advantage to the premade team, no ifs, ands, or buts about it. No one, including me, wants to be at a disadvantage before they even begin. I'll take a fair fight over a disadvantage any day. I WILL leave against a premade before it starts, if I think I can get in a pure pickup game, as premade teams are not fair (i.e. one team starts with an advantage). Good teamwork and communication is king in this game...
That being said, PM teams are not unbeatable, nor is the advantage impossible to overcome. Many matches are not "fair" for one reason or another, but still worth playing. I have even beaten a 3v3 premade team with one good partner and a leaver/AI. But I can understand not wanting to begin a game already behind the curve, just as much as I can understand wanting to start it ahead of the curve (by making a premade).