If you have 300,000 subscribers who play for an average of an hour or two a day you need 12-24,000, even assuming everyone playing is playing a game which requires a single high powered PC to run (where in reality the likelihood is that many of those will be 'casual' games, several of which could run on a single machine).
Very true however the value in this to the gamer is to remove the cost of having a specific gaming platform (console) or have a high cost PC. Casual games are quite possible on the current series of Apple/Google/MS mobile platforms so it doesn't make sense for the majority of users. Those that causal game often do this on the move - so gaming whilst commuting on the train (with really bad stable mobile coverage).
I can see the first MMORPG done like this for iPad.
However todo this properly, the game engine itself will need rearchitecting to have banks of specialist graphics renderers, game loop servers, specialist physics engines, user profile data bases etc rather than having 30,000 individual PCs executing off-the-shelf game installations (which render, control the game loop, store the user profile etc). Then the system can assign resources as needed.. the result would be games that are far greater than a standard PC game..