Really, that's a marketing gimmick more than anything. Sure they can call it whatever they want, but it still fits the definition of a "MMO" and everyone else is completely in the right to refer to it as such (and most do). People come up with their own wonky categories to put their own stuff in to make it stand out more. I notice it the most with crazy music genres.
But, really, this nitpicking is getting away from the point. Is Guild Wars really so far removed from the "MMORPG" genre that it doesn't count as an example of how a PvP focused MMO could be successful?
edit: Even if you don't consider it a MMO, it still teaches a lot of lessons that a PvP focused MMO could learn.
1) Don't nerf PvP for PvE, and vice versa. Split functionality of the skills if you have to.
2) Let people jump right into the PvP from day one and be competitive if that's what they want. GW lets you make a max level PvP only character right from the beginning, and lets you unlock new skills as you PvP. No one has to do anything they don't like just to get to the part of the game they want to enjoy.
3) Put everyone on an equal footing, or very close to it. Someone who spends months on PvE shouldn't have a statistical advantage over PvPers and vice versa. If you can get something through one then you should be able to get it through another. PvPers could unlock all of the armor and weapon modifiers through PvP if they wanted to.
I'd say a large reason why most PvP focused MMOs fail is because they don't come up with suitable solutions to these (and other) issues.



e'know, com'on! Giimmeee di Ke$$$HHHHHH!"