Well, why shan't we talk about both problems:
Class - probably none of them, unlike in previous SD games. You got same weapons for each. If there will be most useless(say, slightly less useful) - it will be operative, as it was in ET & ET:QW.
huh ? Well I would also go for none, but I don't really understand how operatives in ETQW would be a little less useful... I mean, they put the Radar ! I can't count the number of times I had to respawn as a covert ops / infiltrator because the actual operatives wouldn't do their main support job (radar), and only happy-camper-snipe. And then, infiltrate in synchronisation with attacking squad to wreak havoc in defense just before the assault...
The class I play the less in ETQW is probably the soldier, but I think all of them are useful, because that's how the game was built. I guess it really depends on the game experience of each person.
I think the same for Brink, even though the duality of class + body type enhances the combinations and the difficulty of "balancing the usefulness" of each... They take a lot of time to balance the game, so I think it should turn out good, with no real combination "left out". Maybe some would be felt "slightly less useful", but it will probably be once again because of how they are played by the majority of players...