It's a move that will eventually happen, which I feared for amazing games only providing a single player campaign. When they bring multiplayer to the mix, the focus on the single player aspect will be affected. With Mass Effect 3 - When they say multiplayer, they mean Galaxy at War, a four player co-op that they claim is completely optional.
in the official Bioware forums, Chris Priestly gives details of how Mass Effect 3's multiplayer will affect your single player game. The multiplayer is just another choice it seems that will help you succeed in the war against the Reapers, it's just another option. Ignoring the multiplayer completely as if it's not included is possible and you can still earn the best possible option during the war.
The post also includes a FAQ about Galaxy at War. They know that adding a multiplayer component will make us question the single campaigns quality but four player co-op isn't so bad. if they had some kind of competitive versus modes, then our fear has come true. They claim that they made sure not to compromise the single player campaign but we'll see about that when it ships next year.
In co-op, you won't have four Commander Shepards running around. None of the characters from the Mass Effect universe will make an appearance. Instead, your given the chance to play as different races like the Turians, Krogans, Asari, hopefully that dwarf looking race seen in Mass Effect 2, and many more. You create a completely separate custom character for your co-op experience. There will be character progression for your custom character, but sadly didn't dwell on how deep you can get in customizing your co-op character.
When I first heard about multiplayer actually happening for Mass Effect 3, I felt disappointed. First thing that came in my head was Mass Effect 3 trying to provide a competitive multiplayer experience to go against the likes of Gears 3, Call of Duty, Battlefield 3, and other games with multiplayer. With just four player co-op being their multiplayer feature, I'm ok with it.
But now - whatever issues or problems that Mass Effect 3 will have, people will wonder if these issues could have been addressed if they didn't focus on adding multiplayer. Some games are just meant to be a single player experience, but having the 'complete package' is the it thing nowadays. At the end of the day, Mass Effect 3 is still going to be a day one purchase for me. Need to see how i'll end Shepard's story.
Mass Effect 3 is set for an early 2012 release for Xbox 360, Playstation 3, and PC.