Is this still true? I think not in light of more recent research.
Microsoft researchers haven’t perfected the genie, but they’ve found a tool that can increase your productivity by 9 to 50 percent and make your work day easier. And you can begin using it right away. The researchers conducted user studies that proved the effectiveness of adding a second or even third monitor to your workstation, creating a wide-screen effect. In addition, they found out how the operating system needs to change to accommodate a larger screen area.
via Two Screens Are Better Than One – Microsoft Research.
In my personal path, I went from one, to two and even three monitors before returning to two and settling on a single 27″ monitor. You get some of the benefits of the ‘wide-screen’ effect without much of the productivity killing distraction.