The way I feel about this is I agree with Yahtzee's very incomplete thought. A game is crap if it can't stand on its own as a single player game. BUT if that game is centered around a multiplayer experience, then there is nothing to complain about. TF2 is possibly the best Multiplayer game I've played, and yet playing by yourself only recently became an option.
Then you have games like UT3 where the game is meant to be a multiplayer game, but they tacked on a cheesy but challenging campaign mode. I didn't buy UT3 for it's campaign, but I did play the hell out of it.
THEN you have games like Halo 3 and MW2 (I'm pretty sure the quote everyone is getting from Yahtzee came from one of these games). I've not played either because of the warnings of a poor single player experience, but had I bought either, it would have been based on the expectations of CoD 4 and Halo 1/2. Both games had a great single player with multiplayer as an added bonus. It was only after those games became a mad success in multiplayer that the companies that made them decided to ditch a worthy single player experience and just give people more multiplayer. This is where the problem lies because it has nothing to do with making a game good and everything with making a game fun.
Fun games are plaguing the industry right now because everyone can make a game fun, but no effort has to go into making a game good. Compare the effort that has to go into making the games Bioshock and Team Fortress 2. I love both games to death, but Bioshock has a great story and well written characters. TF2 just has some well drawn characters, but we are left to fill in the blanks ourselves (TF2 may be a bad example here because I enjoy this about TF2, but I'm trying to highlight the lack of writing. Other MP games aren't so attractive or deeper than "shoot other team"). If all games go the TF2 route, we might have some fun games for a while, but as others pointed out, gaming communities move on to the next hottest thing, and then this "fun" game is left in the dust. Nothing about these MP only games are good, just fun. Giving game makers a pass based solely on great MP and shallow SP is poor judgment because they are going to just keep doing it. If they want to make MP games, go ahead, but then they need to spawn a different "Deathmatch" series instead of bastardizing the Single Player line (Which is something I give Quake 3 crap about to this day. What the hell was wrong with calling it Quake Online?).