Page--everyone has their own reasons for it, and their own set up. Some do two monitors showing the full forward view and one on each side. Others use a second monitor to display instruments...it really depends on your own desires and needs.
There are some guys, like
www.projectmagenta.com, who do a full scale cockpit simulator, and they have monitor after monitor after monitor after monitor...
Personally, I want two monitors to do a full panel full front window shot.