I felt like that, that why would I want to pay for a game that I've already paid for? However, I know a lot of guys at work who play World of Warcraft, and that's all they talk about. WoW this, WoW that, you get the idea. So when PCGamer included a 2-week trial a couple of months ago, I checked it out, and I have to admit it was pretty fun. The world was MASSIVE compared with other computer RPG's. Plus there were several patches released in just the two weeks I played it, including a huge one that added new zones to the world. I was impressed.
Thing is, I'm not much for sword and scorcery stuff anymore (stopped playing D&D in jr. high, thank you). But I started to warm up to the idea that a huge game that is heavily supported could be worth the $15/month, at least for a few months. Fate heard me say that, and the next month in PCGamer, they included a 3-week free trial of City of Heroes. I checked it out, and now I subscribe. I can't say I'm going to stick with it for the long term, but for now it's pretty fun.
So I guess the moral of the story is I suggest you search out a free trial of whatever game interests you. MMO's are better than I thought they were. Besides, the way I figure it, I got 3 weeks free, then I bought the game for $30, which comes with 30 days free. So it seems that even at $15/month, I'll be getting 3.75 months of playtime out of the same money that would buy a normal $50 game (which sometimes lose my interest in less than 3.75 months).