The reason I stay away from online games that require a monthly fee is because I think that, in terms of gameplay, the monthly fee is not worth it.
Yes, the fees does keep the servers online and active 24/7 and so on. But if I'm going to be paying a monthly fee, I expect more than just a server I can log into and play but also improvements or additions to the gameplay as time passes. Most games do not have anything like that however. Look at PSO. It's still the same game now as it was when it first came out, there have been no regular monthly additional gameplay elements to it.
It's just like playing the single player game, except more people are in it for some reason. But that's not to say PSO isn't a fun game, I really loved it actually when I played 4 split screen with my friends.