Bioware on why Mass Effect 2 doesn't have co-op, yet how they recognize its importance.
"We consider all features, and we recognize at Bioware recognize the benefits and the fun [of co-op]. Most of us are co-op players ourselves. We felt that if we could do it, and do it really really well, it would be in the game. We had to look at where we've taken the direction of the game so far and how [co-op] would affect things. We didn't want to just do it sort of good, people would recognize that it's just a bullet point." - Mac Walters - Lead Writer for Mass Effect 2