Game Design: PvP in "Story-Based" Games

[Part of my MMO Design Folder.]

I quite enjoyed playing through single player mode in the Assassin's Creed series as well as the Batman: Arkham series. As you probably know I am always behind in terms of keeping up to date with them because of my spending habits on games (ie. I don't). I keep finding though that some of these games have a small PvP section to them, like bonus content. Sometimes these sections also sound pretty cool.

The problem I'm seeing though, perhaps because I'm on the older games, is that almost no one is playing these modes. Not enough to even start up a match from my personal experience. I have to question then the wisdom in spending resources in developing a part of the game that is surely going to be unplayable over time. Surely it makes more sense to dispose of it and completely focus on the story campaign? I've found cooperative modes often follow the story line as well like in the Sniper Elite: Nazi Zombie Army series so that's not really an issue. If you have allies cool. If you don't you can still go ahead.

It's just the logic of the PvP thing I don't understand. As a mainly co-op player it's quite likely I am mistaken, so I ask you dear reader: In games like the above, how many hours did you spend playing through the story and how many hours did you spend playing in the competitive PvP modes? Surely Call of Duty guys clock more PvP hours than story hours right?