The reason why WoW is Pay-to-Play is because of the massive amount of content patches. New content is released roughly 4 times a year. You buy an expansion, it lasts you 2-2.5 years and during that time there are heaps of content patches added in.
The other reason why I am more than happy to pay the fee per month is that considering how big the game itself is, the fact that they fix any issues with a hot-fix regularly and keep the game (nearly 100%) exploit free and cheat free, all the time. What FPS game does that?
Its not like FPS games where "we are looking at developing a patch in the next few months that will fix the issue where grenades frag 9 guys" is something that is often said. They fix the issue FAST and most of the time its seamless to the end-user (us).
Another thing though, MMO's can be pay-to-play because of how much there is to do. Any idea how long it'll take you to get a max level character of every class in ANY MMO? Then compare that to how long it'll take you to unlock all the weapons in an FPS.
I do not see the feasibility of a subscription based FPS unless the content is updated so regularly, you never run out of thigns to do. Persistence is another reason. Almost like an MMO FPS... But lets be honest, till I see that, I won't believe it'll happen soon (and I mean properly executed).