I prefer only one display. And I will provide reasons for this:
If I have several displays, I have to move my head too much, because I wear glasses. With my LG M228WA 22 inch TFT monitor I feel pretty comfortable, but when I turn the laptop near to it, I have to move head too often. If put two equal sized displays, I still have to move head. My neck will be in pain only after an hour of extensive usage of two displays.
Another reason to not need several physical displays is because I use AmigaOS, where I can switch between different screens with different resolutions by only pressing Amiga+M and Amiga+N to go back to Workbench.
Yet another reason for not having several displays is because I don't like parasitic lights coming from different directions. Even when I don't wear the glasses, which have antireflective layers, the light coming from the display I am not watching at the moment is annoying.
Of course if you are a stock trader and need to watch every change of the indexes in real time and your peripheral vision is quite trained to spot every change, several displays are good, because this will help your job.