Does every game have to stand on single player alone? No.
But for F's sake, let me know if you're planning on half-assing it. Most of these "mostly multiplayer" games try to sell me on them by barely mentioning multiplayer and advertising their single-player elements.
You say "people who play Modern Warfare 3 mostly play the multiplayer, so their pathetic single-player can be excused." But, how am I supposed to know that? I played Modern Warfare 1 for its single-player, it was short (about 12 hours, cuz I suck at shooters)but it was great. Subsequent installments have been terrible for me, Black Ops was perhaps the worst shooter I've ever played - dying a billion times trying to get through a bombed out trench because they magically spawned enemies on all sides without telling anyone - then when I finally made it to the other side they shot me dead in a cut scene and even though I spent hours dying and respawning, it was still criminally short.
Thing is, if I hadn't suffered through a number of awful installments - paying 60 bucks for each one - I might get suckered in by their amazing visual effects and think they might have a fun and interesting campaign.