As much as I scoffed at the idea of co-op in the main story, where someone takes control of one of your companions, I would have found it much more interesting and exciting than a dull copy-paste of Mass Effect 3's multiplayer. They could even have given the controlled companions extra dialogue choices and such for the person playing, and other neat bonus features like that.
But of course they wouldn't do that, because there's no easy way to monetize it, and at the end of the day that's what the multiplayer mode is really for. To make more money through microtransactions, not to actually provide a new gameplay experience.
Even if it could have been "fun" to hace a co-op on the main story, as you tell it, giving more options/extra dialogue choices for the controlled companions, that would have ruined it, or at least give a big frown, to people like me, who enjoy their completionist playthrough *alone* and with our own times.
Just knowning that there are more options, more knowledge, more choice, which is accessible only if you get someone along with you to play, would make the main game (single player campaign) feel uncomfortable for who doesn't want to share their playthrough with others.
On the other hand, I think that a co-op mode like this, would be the only viable way, which also makes sense in a story setting, to introduce multiplayer to this game.
I, that don't like multiplayer, enjoyed the ME3 co-op mode, so I kinda look forward to this.