If the developers didn't waste their time on multiplay, they could put more resources into developing a more challenging AI for the singleplayer.
A good campaign with good AI and lots of content will beat the multiplay experience everytime, in my opinion.
I find multiplay to be, overall, a predictable experience everytime. I feel that far too many developers, these days, use multiplayer as a cheap excuse for not designing an immersive, entertaining amount of single player content. This, unfortunately, is a trend that we see more and more with the release of new titles.
Often it's a very short singleplayer campaign, with a handful of missions that you can complete in 10 hours. When people complain that the game was too short, there's always someone around who will say, "Hey, now it's time to try multiplay". Loads of hours of gameplay... hmmm?
Give me a good singleplayer experience anyday over multiplay content. I enjoy multiplay, but not as much as I enjoy the singleplayer experience.