So once I beat Singleplayer as Multiplayer there's really no point to playing SP-MP enemy more, because I already know the whole story line, etc?
In a sense yes. Once you beat the story you will know the story of the game. But you play multiplayer and co-op for the experiences and the fun of playing. And example would be Borderlands. I have played that game soooo many times, I know all of the story and most side quests like the back of my hand. But I still play it even to this day. Because things changed, it is still fun to play even with beating it. Paul Wedgewood said that "In Brink the story changes with each time you play. You will have different experiences with each playthrough."
And There are also 2 storylines: One for the Resistance and one for Security.