FlyingSquirrel wrote...
The only logical reasons I could see for picking Morinth are: (1) a Renegade Shepard might be afraid that Samara will turn against him/her once the mission is over; or (2) a Paragon Shepard might decide that Samara is wrong to try to kill Morinth. The "usefulness" dialogue could, I guess, be in line with Reason #1, but it would be a rather oblique way to express it. It doesn't fit with Reason #2 at all, and I would not expect a Paragon to help Morinth kill Samara even if (s)he wanted to spare Morinth.
Renegade Shepard is an ****, but he's not a coward. If he was, he wouldn't be fighting the stuff he does. He's a "get the job done by any means necessary" kind of guy. I don't think he'd hesitate to give his own life if his sacrifice led to (in his words to the reporter) the greater objective. Even if he was worried she'd try to kill him afterwards, would he consider it much of a threat? He picks fights constantly, often times against overwhelming odds. Samara is a force to be reckoned with for sure, but she's no more dangerous than the many, many people and synthetics already trying to kill him. And even if she was... he'd probably just shoot her in the back of the head once the mission was over rather than wait for a fair fight.
The paragon response I can see more possible, since there's some pretty naive paragon responses. Break into Clan Weyrloc's base, kill halfway through, then tell the trash talking krogan that nobody has to get hurt and just give him Maelon? What are you smoking, Shepard? But like you said, the dialogue doesn't reflect that motivation as a possibility.
(In fact, I think there should have been a Paragon interrupt option to stop Samara and keep them both alive. After all, Jack, Miranda, Garrus, and Mordin can all be stopped from executing somebody on their loyalty missions.)
Garrus's still cracks me up. The entire point of the mission was to kill that one guy. So they kill fifty other people to get to him, and then Shepard suddenly realizes the value of life or something. Ha.
Also, why does Morinth's "battle cry" include something like "Fear the power of the Ardat-Yakshi"? Wouldn't the other squad member maybe wonder why "Samara" is shouting that at their enemies? 
I think there was some other dialogue that slipped by where she converses with Grunt and it was written as if Grunt knew who she was. She was also using her normal voice rather than imitating Samara's. I'd guess that originally they were going to have Morinth's path where she wasn't pretending to be Samara. Then they got lazy and decided if they have her go all doppleganger then they don't need to write a bunch of extra stuff.