I really dislike the non-linearity of MMORPGs. I simply need some sort of direction in a game, or else it's not so much of a game but more a part of life. I've had my fair share of history playing MMORPGs (Asheron's Call, Ultima Online) and in the end I felt half the reason I was playing them were because of the built in chat rooms.
Of course, in the case of non-RPGs I would gladly see Nintendo go online. There's an obvious potential to make money here, whether it be through pay to play or simply by selling games. But alongside that, there's an awesome potential to fail horribly.
When they day is done, I'd have to say I'm with Nintendo on this one. I have no compelling need to play online games via console. PC works perfectly fine, and in most cases requires no additional fee.