Take Bioshock 2 and Dead Space 2.
The reason there is an online multiplayer mode is because of you gamers. You demanded that all games have online multiplayer, and that's exactly what you shall get. Enjoy, you reap what you sow.
The first thing I could think of when I heard there was going to be multiplayer was "Why?"
Why, why, why, why.
Why waste your time on what's going to be an inevitably half-assed multiplayer add-on that no one's going to remember at all, when you could use the manpower fine-tuning the single player experience?
But no, every stupid little kid and their brother is whining from here to Timbuktu that they want multiplayer in everything, no matter what kind of game it might be.
Congratulations. You got what you asked for. God, I swear everyone's got freaking ADD or something, you guys can't stand playing a good game by yourself for a few hours, when everything else has multiplayer already.