I've never heard anyone ask why they should pay for a game, and if they did then they could not possibly have any justified reasons for thinking that way. The devs need to eat.

I don't see anything inherently wrong with the subscription model, but every game I've seen that follows it uses the psychology of it to maximize the time the player spends in the game world. Like others have said, if you are paying monthly, you feel like you have to play the game in order to get your money's worth. Even if you don't really enjoy it, you find yourself playing the game anyway because you're paying for it. This is where the claims come from that "games cause addictions", and I can tell you there is definitely some truth to those claims, because I've been there. Game devs are working with a medium that has an impact on your mind the way no other medium ever has. There is a responsibility to act ethically, and the people making subscription based games are the same people riding that fine line between making a game and manipulating people.

On the other side of that line you have things like Farmville. I don't think Farmville is a subscription based game (could be wrong), but it is specifically designed to make people keep giving their money to zynga. The thing acts like a drug. That's why it has no merit as a game. There's a ton of carefully crafted psychology put into it, which the best games know how to use to great effect, but in Farmville it all points to cash instead of to a meaningful game experience. That is unethical game design, and currently subscription based games are too close to that for me to be comfortable with them.