Finally someone who's got it right! :)
Well, Dead Space has always been about the Singleplayer game.
I think the Multiplayer is just there to kind of Flesh out the setting, much like the movie was for the first one.
In Multiplayer, you have to play as four security officers (Or Engineers, I forget which) and build certain items to stop the necromorphs, like a bomb and such. Of course, we already know that those kind of things don't really work that well, but still..it's just there to show, I think, that the people on the sprawl were at least trying to stop the outbreak without Isaac's help, which shows that at least not everyone was useless when it happened.