I disagree with Yahtzee that a game has to be able to stand on single player alone. That may be true of games focused on single player, but I wouldn't even want a campaign for an arena shooter like Quake III or the Unreal Tournament series, nor for something else that is equally multiplayer focused, like TF2. As a message to Yahtzee and all of his followers: if you don't like multiplayer, these multiplayer focused games are not for you. Go play Halflife, Bioshock, F.E.A.R., or any number of other games that are focused on the singleplayer, and have excellent campaigns, and leave the multiplayer games for the people who actually like playing against other human beings.
I personally consider it much better to get an incredible multiplayer experience than a game that has a campaign, but is mediocre as a whole. However, I occasionally play other games for the single player; for example, I'm in the middle of my first Half Life 2 playthrough at the moment. That said, I find online FPS games uniquely suited to a "pick up and play" playstyle. It's true that it takes a significant investment of time to get good at most shooters, but when that investment can be spread out over hundreds of sessions that last from about 10 minutes to however long you have time for a piece, it becomes much more attractive to people like me, who don't always have a big enough block of time to get sucked into a singleplayer game, even one that only has a 7 or 8 hour campaign. Other genres can be the other way around, with a single player game easily spread over dozens of short sessions, but a multiplayer game pretty much requiring an entire afternoon devoted to a single match *cough*4Xgames*cough*.
With an FPS, I'm more likely to put in 100+ hours on the multiplayer than the 10 or so it takes to beat the campaign, but this does not mean that I don't play the campaign; it just has to be a really good campaign for me to finish it. It also doesn't mean that I'll always spend more time on multiplayer than on singleplayer; for example, F.E.A.R. has a very good campaign, but the multiplayer was a severe disappointment. I guess my vote for the poll would be "it depends on the game," which really needs to be added to it.