"Why would you want to limit yourself to only one screen?"
Desktop real estate? How about a laptop with support for only a single display?
You need to provide a citation to back up your claim, I don't believe it except for specific cases (e.g. CAD or DTP). How do multiple screens help productivity for a typical user who does email, or word processing, or spreadsheet, or even web browsing (where a multiple document interface is near ubiquitous).