Blimey between Xbox Live Gold and this that would be a bitch :s
Saying that i already pay for a wow subscription so i guess its not a deal different.
As Egg pointed out, the reason it works for MMOs is they generally have regular updates to include large new content updates, areas, races, weapons/armors, quests, level caps etc etc.
It makes no apparent sense, they have been using the same business model for years with FPS etc, the difference now is that we are coming out of a recession, consumer spending is down and these companies are looking for more ways to address this fact, now everyone is hooked on the Multiplayer market its only apparently "right" for them to squeeze free money out of that, its presumable you wont get anymore than you are now, except if your lucky they might throw an item shop into the mix as well to squeeze further revenue.
This argument is invalid though, the console generation will just pay it without argument, gone are the days of well priced gaming, in 2010 and beyond its going to be an expensive hobby.