Casey Hudson has confirmed on Twitter that it's co-op, which is better for the series than competitive MP. Also bear in mind that the game was gonna be released this fall, but got pushed to spring. that fact says to me in my incredibly overoptimistic state of mind that they probably decided to include multiplayer and were like "we need more time, let's push back the release", so hopefully it'll be not a huge detraction from the single-player, if any. Moreover, a Mass Effect game's singleplayer campaign with some detractions is still going to be far better than most other games' single-player campaigns.