There needs to be an arranged team and random team ladder. There's a market for both playstyles. Some people love to be able to get with their buddies on ventrilo and coordinate their attacks. This is gaming at its finest. However, there are also lots of people that just want to be able to pick up and play without having to rely on others and "find friends" in order to compete.
WC3 had a solo ladder, random team ladder, and arranged teams ladder. This worked the best.
I know that I will probably quit Demigod if I find I "have to" make friends just in order to enjoy myself on Pantheon. I'm too old, too busy, and too engaged with my family to try to create a "social life" in an online gamespace.
Furthermore, my experience with WoW arenas has taught me that teams that are not made of real life friends are extremely utilitarian and perfunctory. A few bad games and one dude decides to quit because he is too immature to handle the learning curve. We don't need any of that here.
Bottom line: If you introduce arranged teams, you have to have a random team ladder. If you have just one or the other, then you will limit the player base and overall popular success of the game.
P.S. I am a ranked player on Azeroth for random teams in WC3FT and I find that system, though with flaws, works well. It's true, you usually get at least one noob on your team but so does the other side. Finding out who that noob is and exploiting that weakness is part of the art of winning random team games. So, on one hand, I love to be part of a team effort. On the other, I don't have the stomach to try to "make friends" with other gamers in order to compete. That's just me. SD and GPG don't have to cater just to my playstyle, however they need to keep in mind there are a lot of people that like team play but just don't want the social overhead of being on arranged team.