I think balance is much more important in an MMO than it is in a single player game. And FAR more important in PvP games. The problem with ignoring balance in an MMO is that so many players seek out the most efficient AT and builds, and if they are demonstrable better it ultimately leads to the exclusion of everything else. Part of the dev team's job is to ensure every AT with any combination of powersets will have a place at the table. No one wants to be told "Oh, you're an SS Brute? Sorry, gotta be Titan Weapon, otherwise you'll just slow us down" when joining a team or TF.
How a dev team balances and what they consider to be fun is of course open to interpretation. I don't always agree with what the dev team here does, but I understand why they do it, and have never once believed they actively seek to make the game less fun. There are only two devs of this game that did this, Jack and Castle. And they are long gone.