MMO on any system would need its own servers and backend, at most it could tap into your friends list on top of its own stuff if it was on another system, give it a rest guys. Whatever you think of Nintendo's online service, or anyone else's is completely irrelevant if it's really an MMO and not something closer to Monster Hunter or whatever.
It's still a weird choice since MMOs are meant to last years and Wii is nearing its end, but whatever, if the game is successful (it will be) they could port it to any other system with cross platform play to keep it alive further.
Also, monthly fees? I hope there's a single player mode on top of that if so...