Bodycount creative director Stuart Black is a self-professed "big fan" of co-op, and he badly wanted to have the game's entire campaign playable cooperatively. But the story had other ideas:
It became really obvious -- and it pained me -- that there were key moments, two or three key moments, where it was absolutely vital that you were by yourself to feel that sense of isolation. [...] And I was really like, 'Oh no, I can't believe this!' and I tried to think, 'How can I get around this?' and it was like, story-wise...I can't. I have to have [those moments] that were too important to drop. So that was a bit of a bastard.
Still, Black was determined to include co-op in Bodycount, and his solution was to have distinct full-scale co-op missions, like the scenarios in Uncharted 2 or the Spec Ops mode in Modern Warfare 2. (He noted that the co-op missions in Bodycount will be longer than the average Spec Ops jaunt.) And the story offered an opportunity to use co-op as a way to expand on the narrative:
...there's a lot of story about what the Network and the Target are doing that we can't tell -- because he [the game's protagonist, Jackson Delgado] doesn't know it. So we thought [that] it would be quite nice in co-op mode, then, if we can't do it in part of the main campaign, to set [the co-op] in the parts of the world where Jack goes, but set it before and after he's been there.
In the single-player campaign, Jack might not have a lot of information about a location that he's visiting. But the co-op missions -- which you play through as a generic Network or Target (enemy) agent -- can provide backstory to fill in the details.
There's still more to be seen from my interview with Black, so keep checking back in.