My only real multi player experience is from WoW and I have to say it was awesome!
Unfortunately the more casual the game became the less enjoyable the game play was. I've met two people that became my RL friends in there but later people started to be rude, impatient and intolerant so I stopped playing.
In general - if the game isn't MMO or shooter (like Borderlands for example) where multi/co op play actually works then fine. But if the game is "solo" like ME or DA series then adding co-op integrated with main story just destroys the game. Usually games like that have watered down story, you can't really have proper dialagues and role play elements cause others may have other opinion on the choices made and so on.
If MEA will have multi tacked on like ME3 or DAI then fine, I'm not a big fan of this but OK, I'll deal with it.
But if we will have forced or optional co-op within the main story then I'll pass.