I disagree, that the Source Hunters didn't get any depth over of the course of the game. I would even say, that they got more personality than an Commander Shepard over the course of 3 games for example and that with a simple trick: every decision I took, they earned an point on an personality aspect so after I while I played them as the characters they were. Not only because of roleplaying aspects, but because the gamemechanics supported this kind of characters they were.
For me this is part of actual roleplaying: Not only choosing an answer but also staying in-character. In ME people I know often choose an answer not because it fitted Shepards personality, but they wanted the approval of their companions, so Shepard stayed a blank and didn't became a fully realised character.
In Original Sin I know now, that my righteous and law-abiding Knight is a vegetarian (because it would be cruel to eat somebody you can talk with) and that he is without fear and that my female Wizard isn't above stealing for the greater good, while being suspicious because the game rewarded me for their decisions.
So the system worked in SP, although I would have liked some more automatism during conversation. But if they balance this out in the sequel, this could work even better. But I have to agree, that it depends on the quality of the actual dialogues in the game. Harebrained Studios showed how much personality you could give an player character with just a few words, to mention a more modern example.