This seems very non-practical if you actually want to keep customers. In fact a lot of new MMOs are having to go to a Free-to-Play model just to get people "in the door" so to speak; because WoW is dominating the market and people are not willing to have more than one monthly fee for gaming.
It will probably be a self correcting problem, as most people will go to the cheapest most popular solution for their gaming enjoyment which would not include online fees on top of the already high purchase price (competition would leverage their free online play to win customers to their side).