15 a month is less than what most people spend on free to play games...that are free to play.
Yeah. I never really found the free to play model appealing-- in fact, I have serious doubts that a game can remain free to play for any length of time and not destroy itself.
With a monthly fee type like WoW, the incentive is for the devs to make the game the best it can be in order to gain new subs and retain the old ones. They miss that target quite often, but at least that's where the pressure is. They don't need to worry about the finances in anything but an abstract way-- if they make a great game, the finances will follow. They can just work on making the game great and let the business end of the company worry about the finances.
A free to play, though, isn't meant to be all that good for the free player... it's meant to give a glimpse of some fun you might have if you pay, but to kinda suck if you don't. Instead of trying to think of ways to make the game fun and interesting, the devs are trying to work in all these little ways to nickel and dime you, and to make sure the gaming experience is inferior if you don't pay.
A lot of game devs say they're aware of that problem, and they promise to be diligent in making sure the game doesn't go in that direction... maybe they say the benefits of paying won't make your character more powerful, but will grant you things like special mounts or vanity items, or to make travel faster and easier, etc. while leaving combat and competitive gameplay alone. As time passes, though, the thought of all that extra money they could be making if the free version was just a little bit less useful, if the paid version gave you just a little bit of a power-up... and then you're on the road to "pay to win," which doesn't make anyone happy.
If a person pays and wins, people will carp at him for buying his victory; if he manages to win without paying, people will assume he bought it and still carp at him for buying victory. If he loses, he resents the people who beat him, which he assumes happened because they bought their victory, which he tells himself a REAL player would never do. This could be in PVP, in dungeons or raids on DPS or HPS meters, you name it. Gaming is competitive, and anytime you have the option buy an edge against the competition, it's going to cause resentment, whether or not you actually make use of that option.
Anything short of pay to win is unlikely to bring in enough money to make the game profitable. There will be some sales, certainly, but if it doesn't affect competition, that will be the exception rather than the rule.
Put another way: The more usable the game is for free players, the more of them there will be, naturally; the idea would be that having lots of free players will bring in the subset who will pay. On the other hand, the more usable the game is for free players, the less likely they are to ever become paid players. If the devs see a great percentage of players don't pay, that's bound to cause resentment against the "freeloaders," and the odds that the devs will end up trying to shaft the free players (who are seen as parasites that it would not be a bad thing to drive away, if they don't pay up) to make them pay up will increase. When you're trying to shaft the majority of your player base, it's not good for the long-term survival of the game. If all of your in-game friends leave, you're more likely to leave. The freeloader that gets run off may have several paying friends who don't want to play without him.
That's why I don't involve myself in any free to play games. I don't see how it can work, and I know that even if the developer promises never to let the game devolve into pay to win, that could change tomorrow-- and I don't want to waste my time with it if the odds are that it will change into a game I never would have started if I'd known what it was going to become.
It's unfortunate that the entire MMO market has fallen for free to play. Now people expect it; the idea of paying a sub fee offends them, even if it is, as you mentioned, cheaper than what they would have spent on it otherwise.