I just don't understand the big deal. The missions last no more than 15-20 minutes each and I've never been hampered because I chose a certain team member over someone else. In some areas you can walk back to the Normandy, but in some you take a shuttle down to the mission. To get a new player you would have to run all the way back to the beginning and then take the shuttle back to the Normandy. IMO they did it this way so it wouldn't break the immersion. Swapping members in and out on a whim would kill it for me.