Defecting to the republic, switching sides, doesn't mean changing professions. He's still a Sith Warrior who works for the republic, even if neither party would openly admit it
Actually I don't agree that Scourge is working for the Republic. I think he's working for himself, for his own survival, and he just happens to be aligned with one particular Jedi to accomplish his goals. But, assuming that he is working for the Republic, he is not the very best Sith Warrior in the galaxy
and is not even considered a hero of the Empire. The player Warrior would destroy Scourge in a fight.
You can argue this all day, you won't get anywhere any more than you would if you were trying to argue that water isn't wet. The fact of the matter is, you are not the person who developed the game. Neither am I. I am telling you exactly what the reasoning behind the developers' decisions were, based on the developer blogs I bothered to read. They may possibly put in Republic Bounty Hunters and Imperial Troopers at some point. But it won't be a defection story line, because that runs contrary to the idea that a player could level a Republic character from 1 to 50 and then level an Imperial character from 1 to 50 and never once run into a duplicated quest.
Now sure, you can argue that all quests boil down to "Deliver this to there" or "kill this many of those" but you'll be doing it alone.