How Essential is Multiplayer to FPS/FPA Games?

[LG[Infinite]]

My question is: how essential do you think multiplayer is for first person shooter and first person adventure games? This has been bothering me for a while now, but it's really come to a head with the prominence of Bioshock and Metroid Prime 3, two excellent FPS/FPA's, neither of which have multiplayer. What I found interesting is that looking through the reviews for both, a consistent complaint about MP3 is its lack of multiplayer, whilst this complaint is seldom, if ever, expressed about Bioshock. Given this apparent lack of symmetry, just how important do you think multiplayer is? Furthermore, what particular features of an FPS/FPA demand it have multiplayer? For example, why was Bioshock not pulled up on lack of multiplayer but Metroid Prime 3 was? Now let me make myself clear, in case anyone wants to start flaming: in no way am I saying that Bioshock sucks - it's obvious that it doesn't. I'm just curious as to why some FPS/FPA games are criticised for lack of multiplayer and others are not, despite their many similarities.