My major arguments against co-op have always been quite simple:
A.) Co-op does affect single player.
Implementing co-op is not as simple as shoving another player character in the role of a squaddie and calling it a day. Unless you want a lame co-op experience, the devs need to find a way to properly scale battles without resorting to bullet sponge enemies. They would also need to design levels with two bodies in mind: put in two turrets instead of one, widening hallways to keep things from getting cramped, etc.. Dead Space 3 had certain areas completely cut off to people playing alone, because some mechanics simply wouldn't work for soloists.
B.)There needs to be good justification.
"But I like doing stuff with my friends more!" isn't a very good excuse to devote time and effort towards a central mechanic like co-op. Though I may decry the detriments of Dead Space 3's co-op, I'll at least admit that they had a decent gameplay justification. The concept of one player seeing an entirely different game because of their insanity is interesting and relatively well executed in the game itself. Mass Effect, however, doesn't seem to scream "I need co-op," in any form or fashion. In fact, I'd argue that the single player power fantasy is incompatible with the concept of integrated multiplayer.
C.) It can't work in Mass Effect without some serious tweaking.
As I said, it isn't as easy as flipping a switch, but Mass Effect in particular would take a lot of tweaking. Co-op games can't pause or slow down time, so powers like Adrenaline Rush and, more importantly, the pause menu would have to be reworked. Fixing powers might be simple enough, but removing the ability to pause and use powers (many more than the 3 ME3MP offers) and control squad mates won't be fixed so easily. And even if it is "fixed," I'm not sure how I'd feel about a Mass Effect without one of its core tenets.
D.) The gameplay still hasn't been perfected.
Mass Effect 3 was fun, but it can be improved, especially with regards to squad control, power variety, and tactics. No team is about to tackle both a major mechanical upgrade and co-op, so I'd rather BioWare spend their time making the game generally more fun, than messing with the mechanics for something that not only isn't something that Mass Effect needs but also something that not everyone really wants.