I don't think they have the stuff to pull it off right any more, but I also just don't like the concept. Having replacements waiting in the wings who could and should have been fighting for you already is just too goofy.
Really depends on the situation.
Here is a scenario I was thinking about: You have one guy as a potential squad-mate, a military man, career solider who deviates from the rules but is otherwise by the books, part of your team from the get go.
Lets say we get first contact, or meet up with a group of a new alien species; the meeting goes wrong for whatever reason. Our military leader kills them in cold blood or something or sees them as hostiles because they attacked first.
Say one of those alien beings was a leader and rabble-rouser who tried to organize them to fight back in self-defense, claiming they were shot on first. Now you can have a potential side-quest scenario on how to make peace; you side with the military you are with and kill this rabble-rouser, or you can side with the rabble-rouser and fight your own troops.
You had the military guy as a long range soldier type in your party, and if you side against them, you lose him because he dies and the alien rabble-rouser joins you instead; a gesture of good faith for sparing his people. Now you hear his side of the story and his personal beliefs.
Maybe this even comes to play later on; tensions boil and having one of those characters can be used for, or against, diffusing a situation between the military and the aliens.
Or maybe a happy ending exists, where both sides reconcile. Then you pick one of the characters to continue on with you, while the other becomes a quest giver or background role, like Ashley/Kaiden can be in Mass Effect 3.
This is obvious a fan-styled scenario, but such a situation makes sense; it changes the narrative tone, some missions, is a complete side-quest styled part of the game, and can deepen the characterization of not only the protagonist, but the two characters here representing their specific sides.