MMORPGs get away with a monthly fee because their servers are huge, and they regularly update for many years. Look at how much WoW has changed since release (even ignoring the expansions, the UI and feature list has changed radically from patches) and other MMOs like Eve Online rely solely on patches to expand. If IW is willing to stick with MW2 for several years, and add maps on a monthly basis, it might be worth it. But if they're just going to give you some nice stat tracking then pump out another sequel, an online service won't work.