I have three screens on my system here. It used to be two 21 inch and one 28 inch. Now its two 28 inch and one 21 inch. (took the other 21 inch screen to my other office today).
Why? Well, have you seen the images of SS Pro?
Also, I produce a lot of computer programs and do a huge amount of writting (books, magazine articles, web pages, systems fo rclients, etc) as part of my work. Having several large screens means that I can switch between things very easily.