I'm not opposed to a Bioware or Mass Effect specific multiplayer experience, what I AM concerned with is them doing it in ME3. The company needs to do everything in its power to make it as epic an experience as is humanly possible, no one needs the distraction of integrating multiplayer and balancing it right now. Seeing it as a DLC or part of a stand-alone game later in the ME series, I'd be damn excited for it, especially if it is co-op and still has a great integrated story (A team of Spectres who can talk individually or together to people? WANT IT).
I'm not saying this would make me cancel my pre-order or anything, nothing could stop me, BUT, I would be very concerned and those concerns would not be alleviated until I finished the game. Plus, if I felt the game WAS a bit lacking, I know my initial feeling would be to blame the multiplayer even if it wasn't necessarily to blame.
So for now, I hope this is just a rumor.