If people can get a $60 singleplayer-only game without complaining, then they probably shouldn't complain about a $60 multiplayer-only gamer. Not every singleplayer game needs an online versus mode, and not every online shooter needs a singleplayer campaign. I wouldn't advocate against the inclusion of any of these, so long as neither feels contrived or detrimented.
But that's just for the now. When servers go down, the shit hits the fan for MP only games. Fun while they lasted. That's the argument against them.
bet: lost