So i take it that every player has to be able to play every class to play in a team environment? Your not going to be able jump into a unknown match and say, "I'm playing Engineer and I don't care that we already have 6." That'll hopefully never be the case but you get my point? Will the community be willing to change their class to what is needed or like every other game will they all go off on their own?
I think you're going to find that there will be room for specialists and utility players, so most will be able to pick a player from the start and run with it. There will certainly be players that aren't interested in switching classes, but the more we learn about the various abilities the more likely it is that we will be able to build our "specialists" as a split player capable of running well for two classes and not just one.
I've played PC, Wii, and PS3 online and while you certainly get loners, I rarely see games unfold into complete anarchy. Most players even without communication will gravitate to the pack so as long as there are players that understand what to do and where to go the rest will follow.