I get the feeling that if something like this were to be implemented, it would be the beginning of a downward spiral. At first, charging a subscription for online multiplayer might seem innocent enough, but then some publishers will get the idea to start forcing people to use their subscriptions by gimping the single-player mode even further, maybe even removing it entirely. Then they might even do away with splitscreen or local multiplayer as well, meaning you would now have to pay $60 for an online-only game that you would also have to pay a regular subscription for, and that's not counting things like map packs and new content.
Yeah, this is really the worst-case scenario here, but knowing certain publishers, it wouldn't surprise me if this ends up being the route they take.
