Sure, it takes two to tango, but it only takes one gun to kill both dancers. Add in another gun and, well, it's overkill (sorry, dancers!). But for the purpose of taking down multiple enemies (say vengeful family members of said dead dancers), having a co-op partner is definitely handy.
Army of Two: The 40th Day is all about that -- having a bud by your side during the heat of battle. But it seems like EA is trying to make killing dudes with your friend mean something more. In fact, in a game where you play as two badass mercenaries, they're trying to incorporate something seemingly ridiculous in the killing trade: morals.