I have to disagree. The problem I have with many modern shooters is that they proudly display single player mode on the box as a major selling point on par with its multiplayer, yet it often results in a half-baked campaign that only lasts four hours. There's no way in hell they'll give multiplayer the shaft because that's what's popular nowadays, but single player campaigns are viewed as little more than a marketing ploy to sucker solo players into making a purchase. Therefore they're far more likely to screw over single player mode.
If developers don't have the time and/or resources to make a decent single player mode, then it should be axed entirely. Indeed, some of my all time favorite shooters have taken this route, such as Team Fortress 2 and Killing Floor. Conversely, if they're going to have a tacked-on multiplayer, then maybe they shouldn't bother with it in the first place.
The way I see it, multiplayer gamers have never had it better. In the past it was multiplayer that got the shaft because online gaming was still in its infancy and didn't have a large following. You were lucky if you got four square maps with a few floating platforms that supported 8 players on a 56k connection. And players were happy with that, dern it!
*waves cane*
EDIT: Also, consider this: any multiplayer game, no matter how popular at launch, will eventually "die." Players will move on to newer games and/or hardware, meaning an ever-dwindling population on the servers. Many old games now have no multiplayer whatsoever outside of LAN matches because the publishers shut down the servers. Given the choice, I'd rather have a shooter with a decent single player mode I can replay twenty years later as opposed to finding an empty wasteland because it was multiplayer only.