Gameplay > Graphics.
Since the beginning of CoDs competitive gameplay, it's well known and common sense to strip the game of it's eye candy in order to gain advantages.
id have agreed in the days of cs and its source varient when games were released on the bleeding egde of the hardware available, even cod 2 but for the most part gamers now own computers that can already max the game fluidly without input lag. today there isnt a need.
'edit'
what he means is cod players always have the settinsg as low as possible
a) there's hopefully less object/less opaque smoke etc so they can see people better.
b) they all like to exploit the engine glitches at certain fps's.
never noticed any issues, maybe all in there mind?