Problem with this article is that MW2 IS gonna have Co op, or at least thats what i understand anyway.
Its just that its gonna be like Resistance 2 in that it'll be seperate missions rather than going through the campaign apparently.
I do prefer it that way cos they you dont feel guilty about leaving after an hour or so when everyone else wants to go through the full game.
To answer the question though, well, multiplayer cant hurt can it? As long as the single player stays good then it just makes the game last longer.