BioShock 2 was a prime example of a great franchise being spread too thin. Technically, the game was worked on by two different developers; one for single and multiplayer respectfully. Yet in my eyes that's still wasted time and money that could have bettered the admittedly disappointing single player segment. Instead of having yet another online game mimicking Call of Duty as closely as possible without being sued, the offline game could have been improved upon.
Another aspect to consider is the fact that Mass Effect is a third-person shooter. Games like SOCOM and Gears of War are fantastic franchises in regards to multiplayer, but Mass Effect simply isn't cut out for online modes unless there is some major changes to the gameplay. The main concern is in terms of powers and skills. In the single player, Commander Shepard and his team are intentionally overpowered compared to the typical grunt- that's what makes the game fun. In a theoretical online versus gametype, how would Biotics and Engineering work? Would Soldiers be the only class worth playing, or even the only class you can play as? In term's of balancing, if high level characters were given access to more powerful attacks how could low level gamers compete? I can't imagine a fair playing field when the possibility of being thrown 100 yards into lava due to a biotic attack is an immediate threat.
The point I'm getting at is that if Mass Effect 3 were to have an online gametype, it would either be a broken abomination or a simplistic shooter. Well, the third choice is they make it completely different than the single player, to the point of it being treated as a completely different game (Conker: Live and Reloaded). Ideally, we would all appreciate a well-made multiplayer segment; co-op could do well if it supported unique missions. And versus games could work if enough time was put into it. However if any of us want to play Mass Effect by next year or sooner, don't even think about multiplayer. For a game so heavily rooted in single player story, I can't imagine anyone would feel ripped off at the lack of multiplayer.