The first ME game to have multiplayer was also the first ME game that I didn't replay the single player campaign. This fact, combined with other examples of franchise that I feel have become diluted by forcing multiplayer into them, is why I feel the way I do about it. Also, as I said, the market isn't exactly in need of more multiplayer shooters.
The idea of a separate specifically multiplayer game at a cheaper price is a good one however. My gut is telling me that a ME MMO is in the works anyway. We just haven't heard about it because the SWTOR fiasco is forcing them to wait for a better announcement window.
edit: corrected rice/price typo and added my tinfoil hat speculation of the day.