I was reading an article this morning on Joystiq that was discussing the multiplayer mode in Dead Space 2. In that article the author, David Hinkle, makes this statement in his very first sentence: "One thing that hurt the sales of the first Dead Space was its lack of multiplayer." That's a fairly bold position to take as he provides zero information to back this claim up. I'm certainly not in a position to know with 100% certainty either, but I've seen no data that would begin to suggest that the lack of multiplayer is what caused the slow sales of the first game. I think Dave might need to go watch what Doug Stanhope has to say on this topic (with particular attention at the 2:05 mark in the video). 
 
This isn't something that is unique to people who write about games, though there are a lot of bloggers that do this with startling frequency. I'm hesitant to continue this entry myself as I'm starting to fear that I will be proving my own point. I'll have to come back to this later when I have time to back up what I'm saying with the examples that I've seen.