That would be nice. Co-op is a must these days. I don't know how COD4 got away with it...
By making the best single-player campaign ever instead of adding tons of features that are used by a minority? COOP for a FPS is a bit silly unless you have a campaign that has teams all the time.
Co-op would not have worked for COD4 I agree...no vehicle sections, event driven gameplay... but there were sections where I felt like I could use some backup. Like when you walk in to that news station...that was rediculous on veteran. Co-op would have been perfect for times like that.
In any case, Co-op is not a minor feature...It extends the campaign replay value quite a alot...and if its used by a minority, the majority is missing out big time.