Last Month, Fragland.net says (in News: MW2 campaign co-op ruined the experience) Infinity Ward's Rob Bowling has revealed that originally, Call of Duty: Modern Warfare 2 was planned to have a full co-op mode in the main campaign of the game.
“But then there were moments that were really fun with two players, so we took those moments and put them in Spec Ops,” Call of Duty: M...

And Blue from Blue's says (in MW2 Co-op Killed for Killing the "Experience") VG247 quotes Infinity Ward’s Rob Bowling explaining that Call of Duty: Modern Warfare 2 was to contain a full co-op mode, but this was axed for the dedicated co-op structured missions of Spec Ops....