The system the OP suggests eliminates the middle man in real money trading (RMT) and instead of paying the RMT to buy gil, you buy a 30day sub and sell it for gil, and I think that its a pretty nifty idea.

Those efficient at making gil can play the game for free by paying for subs with gil instead of real money if they choose to do so and those that fall on hard times/lose their credit cards/young people with a lot of time and no income can still play the game.

Only downside I can think of is what if RMTs sold gil cheap enough that people would buy the gil from them then turn around and buy subs from the AH with that gil and end up paying less than the official cost to play the game.