I think many 'serious' gamers resent the massive economic impact the yearly blockbusters have on more 'artistic' releases.
Personally, I am a huge fan of CoD1/United Offensive. UO's multiplayer in particular, it brought the CoD formula to huge open tactical maps, with vehicles and objective based play. Buuut.. the UO expansion wasn't made by Infintiy Ward, so say goodbye to all that good stuff..
CoD2 was released and the MP had reverted to rapid respawn killbox maps. I played the epic campaign about fifty times, but the online felt like it had devolved.
I have paid for every CoD title released for PC and will continue to do so, but mostly just for the campaign. To my single player viewpoint, it's not CoD4 multiplayer repeated for four years(even if it is), it's episodic gaming!, with a new crazy action movie to play every fall.
But until they concentrate on co-op (other than stupid zombie mode) I couldn't care less about CoD MP. World at War had a co-op campaign, but that was devolved out from it's sequel as well, though I can't imagine Black Ops being any more buggy and crash-tastic than it already is.
MW3's Spec Ops doesn't seem to be making huge leaps in co-op either, they've added a ubiquitous horde/survival/not-zombie mode but.. meh. Spec Ops could be a standalone co-op game without competitive MP, or update the previous year's MP as a bonus disc like HaloODST/Halo3, and.. um.. actually admit that it's the same game from last year.
I wish I could just buy the campaign half of the game.