Why is it that every time single player games succeed and sequels are made, people demand a co-op? Isn't the point of the single player genre to play by yourself? Go play Call of Duty if you can't live without someone else's mic making static noises through your TV.
The biggest argument in favor of co-op/online interaction is "if you don't like it , don't use it." Yeah, just have a developer add a few lines of code so you can fart around with your buddy and sell all his stuff when he's not around.