co-op mode is a good addition to the game. Its enjoyable to play and attracts non competitive gamers as well. I only played halo BECAUSE it had co-op mode. I wouldnt have played it if it didnt, since i wasnt interested in competitive halo play.
but co-op mode isnt what gets me to install quake 1 once in awhile and have a LAN game with my fellow FPS players. Its the DMs CTFs, TFs and the **** talking... in games like FPS, good SP, story plot and co-op is just icing.
RTCW also had co-op too in xbox. it seems that most fps is only interested in having console gamers play co-op or something.