The argument for "oh, I don't want to have to fight in a party. NPC's will make it more possible to solo," is, essentially, void. If Bioware wanted to make the game so that you didn't have to completely rely on playing with a party, they could modify characters classes and mobs so that it would be possible. You can solo 1-70 in WoW and there aren't any controllable NPC's. Want more party play, but don't want to be dependent on it? Yeah, they can edit characters and mobs for that, too.
I don't see how NPC's would present much of a fun-factor, either. As I've said before, I'm hesitant to believe that Bioware can properly pull off half the features that they claim to incorporate in TOR. Do you honestly think that Bioware is going to be able to make NPC-based stories as deep as K1 and K2 and somehow place them in an MMO? Yeah, it'd be cool, but as Ctrl, etc. says, you'll be sorely disappointed if you're expecting that. Maybe the NPC's will have some depth, but lets be honest: If they're anything like Guild Wars NPC's you'll be more likely to find deep back-stories between yourself and characters on a roleplaying server, where the plot between characters will be your choice.
"Nice to meet you. I'm an assassin.... Whoops. I wasn't supposed to say that."