Why pay a sub? Why play continuously?

The experience itself. The journey. Hanging around with friends, having fun with them, on a regular basis after a hard day's work. MMORPG are a totally different experience than other "shoot and leave" online games, because of the very fact that the world is permanent, and so are its social structures. It's one of these regular things we do every week, if not every day: a bar you like, a friend's you enjoy. MMO players are creatures of habits, which is why it's a very human hobby, despite its being virtual —emotions experienced in an online setting are by no means less "real" than those experienced with a physical object, especially when they involve other people in a social setting.

It's the very notion of "bonding with others" that is at work here. Something that tends to vanish in the current "I want it now - don't care about others" mindset that many people tend to develop these days in most of their activities; but in the end, we all need others to enjoy life. MMO gaming, on a very basic and humble level, is a way to do that, especially for people that don't have the luxury of going out every night —be it because of tight schedules, long commuting times, or even social fears.

That's why you subscribe to a sports club, you pay regularly for drinks in a pub, or you hang on to some online games. The activity, itself, may be more of a pretext than an end; and besides, it's the journey that matters, not the destination.