Well there's the notion that adding MP just because some suit said so, takes away resources from the SP.
In that sense, i understand why people act negative about it. To me, well, i say watch that latest Gametrailers T.V show, and there's this 2K producer saying what he thinks, certain games don't need MP, Mafia 2 doesn't have MP etc
He also says something i've been saying forever, we are reaching a point, where certain genres are starting to all play the same, and the graphics are all at the same level of quality, and developers need to start concentrating on story, because that's what differentiates certain kinds of games.