Right now, I run three displays across two computers. I use one for email, center is main, and left is generally communication. But there are times that I could really use more screen real estate. Communications, email, research, active project, music management, and static data monitoring. As scott said, the process of switching things around on what you have can easily break your focus on what you are doing. I personally could actually see making a six display setup function to it's full capacity and keep me flowing at about max efficiency on a heavy project.
Or, when things are going fast, I could see using it for multiple projects. For example, if I'm watching two helpdesks, researching problems on project A while making progress on project B. As Scott said, technically it is still context switching, but that is the way I see working on multiple things.