Well, I can see both sides. Single player means no jackass teammates or grievers, but you're missing out on a lot of the team work mechanics that make the game fun (not to mention that in single player, when your character dies, that's it; restart the map regardless of who else was still alive).
Campaign is fun the same paragraph, but reverse it a bit so it's about teamwork, but has assholes.
Versus is like... Versus is difficult, sometimes. I don't mean, "It's hard", I mean that it's a rather different mindset to get into. You can play the campaigns or single player and get a really good idea of when, where and how the zombies will attack. Versus, however, removes that knowledge because suddenly, you have Special Infected that will not only hold back and plan, but do so with all kinds of different tricks. I've never seen a Hunter claw swipe someone and incap them in campaign, but you damn well bet it happens a lot in Versus.
And then you have the tactics used for playing as Infected. Very different. Coordinated strikes, waiting for the Survivors to come out of their hiding spot... it's a different mindset.
So, yeah, tl;dr version: Versus and Campaign are very different kinds of games.