I just hope they don't screw things up by making some things only unlockable if you meet certain requirements on Co-op mode. The reason being is that I'm useless at Co-op modes in any game and I'm not sure I'll have anyone to play Co-op with anyway, except online.
It depends on where you stand, I guess. I think it's a good addition, but I don't think that it should be essential that you play it to unlock anything, if you follow me. Perhaps an alternative option to unlocking, say, a powerful gun, that can also be unlocked by other (single player) means.
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