I would definitely play a Pokemon MMOG. I might dislike it after a while, but I would play it until I realized. My main concerns about it would be the community and the use of overpowered and/or legendary pokemon.
In the current games, it is nothing special to have a few (if not all) legendaries in your pokedex. So what makes them legendary? In my mind, they're legendary pretty much because the game tells me that this lugia or this celebi is special. Now if the creators made legendaries virtually impossible to find and defeat/capture, and if there was only, say, one zapdos per server or something, that would make them special. Not to mention it would make the pokemon universe more real to me.
I recently realized that in order to succeed in the pokemon universe (mainly the metagame), you have to think almost entirely in numbers. What effort values will make my togekiss the most effective in battle? What dragon type move has the highest power while still maintaining accuracy? At a certain point it stops being about enjoying the fantasy of running around with these fantastic creatures and starts being about playing to win. I enjoy the fantasy aspect of the pokemon games, but I'm worried that an MMOG would be overrun with people who care nothing for stories and escape and only care about number of wins versus number of losses. I admit that many, if not all, MMOGs have a population of such players; and money is money, so Nintendo wouldn't care; but if a pokemon MMOG happened, I hope that the community isn't full of assholes.
Wow, this got longer than I thought. TL;DR: I'd definitely play, but with reservations about the community and how the established pokemon universe works.