*in short* Campaign is important, I think it (more than multiplayer) presses the gaming industry forward by forcing them to create new ideas that are integral to SP, and will get worked into the multiplayer. BUT, multiplayer has easily evolved enough now that it can often quite easily be considered it's own game.
I have often agreed with a lot of what Yahtzee writes, but I think he takes his avoidance of multiplayer too far really.
The mechanics of a multiplayer game versus it's campaign counterpart (while they CAN be similar) tend to have some pretty distinct changes. The fact alone that you're competing with/against real people, not an a.i is dramatic in itself. For example playing against real people often gets rid of (I believe he phrased it as) "Infinite laser-sight targeting fuck you vision" that enemy NPC's are sometimes blessed with so as to be able to remove your testicles.
Add that there are an infinite number of ways for any multiplayer match to pan out, but you can only play through a singleplayer game so much before you really ARE just knitpicking over which side of the hallway you're going to walk along to get to that door over there.
Singleplayer experience is important because it forces the developer to create a really developed (huzzah) system that relies on MORE than just more real people in a given space, shooting at each other. I think multiplayer continues to force the gaming industry to evolve to stay fresh and interesting, and these ideas begin to form the foundation for a stable multiplayer mode.
Because it's 4, and I'm tired, and it's the internet, I'm going to finish this with a paragraph lacking in detail and say *ahem* Multiplayer should also be looked at as a new game entirely. You really do yourself a disservice by avoiding multiplayer altogether.