Absolutely, MMORPGs have arguably the BEST environment for AI development! Just as you said its because of the plethora of various "play" areas and the vast difference between what one agent (creature) can do and what another cannot.
However, in their current state I think that games like WoW and EQ2 use FSMs for three reasons...
2) Processor Power versus Server Cost
3) Content Development
In argument 1, I don't believe players really want emergent behavior in their encounters. Granted I only played WoW briefly, but alot of the players depend on forming a single strategy to take a raid on and they don't want suprises. I suppose that might be because if a raid fails, it does'nt just hurt the ego of the leader but the whole of 25-40 people can be impacted. Since they're paying monthly, as a business move that wouldn't be very smart to just piss the players off On this point a little further, players who aren't a part of an elite first-encounter group, have the ability to study another guilds strategies so that they may more easily brisk through it. If you're looking at it from an academic stand point, then this is completely irrelevant.
In argument 2, this is an obvious reason. There are hundreds, if not thousands, of AI agents being generated and controlled by a server at any given time. FSMs just allow the CPU to plow through a few statements and determine a behavior, rather than work with something more complex like fuzzy logic or a neural net. Obviously, as processor power increases and server costs decrease I imagine this will become a moot point.
Lastly argument 3... this is pretty straight forward. I just don't believe that after looking at my first 2 statements that producers see much need to invest in an AI programmer like Jeff Orkin. They'd rather focus their efforts (again theorizing here) on developing artwork, stable clients on every possible piece of hardware, server and network code.
Hopefully, I'll continue on the course I'm on. I always thought an MMO that focused on AI would be more interesting then any of the existing ones. Since the game is already persistent, it would just help reinforce that illusion of another world. Imagine seeing a human child NPC one day, and in a few weeks you come back and he's grown up and after exploring the world has picked up a whole set of skills. On another server, he may not grow up (gets killed) etc... This sort of behavior would work really well for cities/towns.
But how would you handle boss encounters or dungeons? Might be interesting to have a dragon that memorizes how players encountered him last (and does he carry that memory to his next respawn?)
So many questions to ask after that point