This all comes down to the fact that the servers cost soooo much money to run. It costs so much money that they need to pass that on to the customers. I understand that, but they make about 5 dollars for every 10 that they get from you. That is how MMO people get rich. You wouldn't normally get 50% profit on a regular game. Its only MMO's where they can get massive profit like that. They also get the profit on a continual basis (every month), which is a really good business decision if it is an addicting game.
The magic formula Addicting Game + Monthly Fee = $$$$ comes to mind when I think of MMO's. MMO's are not worth the money ever, its just that simple. If you want to waste your life away leveling up a character for absolutely no reason, go right ahead. MMO's are really a basic economic principle, like the magic formula above. Blizzard will make insane amounts of money on this game regardless of whether or not it is bad. Because MMO's are a safe genre these days, making an MMO is about as risky as releasing the latest Pro Football game for Xbox.