Good points but I have to disagree with the two first sentences
Shepard didn't stay blank at all. What you seem to miss is that a party RPG is much about group dynamics, just like in real life.
Shepard stayed blank because aside from one or two lines of dialogues, there were no signs of him beeing affected by the horrors around him or what happened to him. He could be an stonecold sociopath, that says things, because he knows that the people around him want to hear it (my personal interpretation of the character), not helped by his dead fish-eyes and his emotionless expression. And besides some few lines, there exists no group-dynamic. Everything is build around Commander Shepard, your companions have no life besides Shepard, who most of the time knows what to say, because there exists most of the time only two lines of dialogue (beeing a jerk or saying what they want to hear). You are Commander Shepard, you are the Center of the Universe. It helps for being self-indulgent, but it doesn't gives you a real character, not helped that there isn't any real personality in your lines.
A character where this worked better, was Mike Thorton in Alpha Protocol. You also has to choose, but he was always some kind of asshole, because every answer of him is writte n as one. You understand much better why his environment is reaction this way, because of this.
But Bioware - Games are most of the time a bad example for real-life group dynamics, because most of their characters are anything, but real. Well written, but not real. I can't lay my finger on what it is, but you always know, that they are just characters, not people. Maybe because most of the time, there is a definitive way, to resolve their quests and at the end of the game they want to kiss your butt, for being such an incredible person (while you look at them with your dead fisheyes).
I think it was better solved in Pillars of Eternity, because alrhough you learned some about your companions, most of the time they stayed for themselves and wanted to be alone. Don't know why, but that's sound a bit more realistic for me, when you put a bunch of misfits together and you have to kill at least two dozen monsters on a daily. After a long day, you want some time for yourself.
That game had also some personality mechanics, which helped you at some points and like in D:OS they helped, because they showed that there were consequences for your decisions.