Games with monthly fees just show that the people that made it are in it for purely monetary reasons. It's like a drug- get your players hooked and the collect from them monthly so they can keep playing.
Look at WoW.
MMOs require a constant team, and the game is constantly updated with new content, and also giant servers are not cheap. They don't just make the game and then sit back and smoke cigars and rake in the free cash, there is lots of work to be done. I would rather pay 15 a month for an MMO that would take me years just to do everything possible in it, and keep getting constant updates for more stuff to do than buy a game every 2 weeks for 60 bucks that I play for a day.