No, no, a thousand times no!!!
Please dont give Bioware the idea to turn Mass Effect into an MMO like they did KOTOR.
Seriously, I guess I would not begrudge a Mass Effect MMO if they brought it out AFTER a great ending single player Mass Effect 3. But I will always be disappointed that there was not a KOTOR 3 to finish up the story line, especially after the rushed and unsatisfying ending to the second game.
Personally though, I will always prefer a single player game to an MMO. I want a game to play on my own, whenever I want, without having to party up or quest endlessly for a story that goes on and on without any resolution.
I dont mean to offend those who like MMOs, they are just not my stlye I guess. If that is what you like, then enjoy.