I started out typing this saying it could be interesting, but as I was typing it I decided it just wouldn't work so had to start over.
First off, Diamond/Pearl/Revolution is about as close as you're going to get. You get to raise your Pokemon, trade, and battle with friends. That's all a Pokemon game is about and making it an MMO wouldn't improve that.
It might be interesting if it were a traditional MMO and your Pokemon actually form your standard party, but that would completely defeat the purpose of an MMO. You would never have to interact with anybody except for battling someone (which you can already do) because you're your own group.
I also thought for a minute that it might be fun if you're the Pokemon or if you get to have a more personal connection with a single Pokemon (like Pikachu) but that would also really defeat the purpose of the game. True, you have 300+ classes which would be neat, but it wouldn't be a Pokemon game and it would fail.
So yeah, no Pokemon MMO needed, they're just going to keep releasing the same exact game with different names.
