I don't need 5 monitors, but I can tell you, it makes my courses a LOT easier. four centos images on my 1080p monitor in runlevel 3, one in runlevel 5 for GUI client on my 1280x1024, my course instructions open in big font on my tv monitor, multi-tab chrome open in both of my remaining monitors to google stuff the course instructions didnt tell me or to get online materials. THAT is a real reason for a 1055t and 16GB of memory. Sure beats the hell out of the Core 2 duo single monitor systems we have at school.
If I gotta do something at school, I steal my neighbour's monitor and do an "mstsc /multimon"
A 6950 1GB serves 3 monitors and could serve one more with a mDP adapter. But past that, the expense of specialized adapters or monitors is too much. My other two are served by my ATI integrated 4290. A low profile passive 6450 will serve more monitors if I can get my hands on more.
When I do my daily stuff, I usually split my 1080p monitor into four explorer windows for quick file browsing in multiple locations. My 768 monitor is best for web browsing and my 1024 is used for my rdp connections, usually to my server (ftp,file,media,apache). The Tv is used for movies and games. The CRT serves for my music playlist and system monitoring tools. It's unfortunate that RDP is either all monitors or just one. Netbook or phone for chat programs.
There are real reasons for 12 monitors. Why else would they build it? From what I gather, financial analysts use a lot and stock people too. That said, I don't know anything about stock markets. I'd also assume some autocad or other design programs. There's advertising markets who need an inexpensive solution to a 200" screen and can't use a projector. There's even the possibility of setting up a single computer to host multiple users across virtual machines, one per monitor, with keyboards and mice dedicated to each VM.
I've personally found multiple smaller resolution monitors alongside a couple 1080p monitors to be most productive. The windows 7 "snap to half" is definitely a nice feature too.
And of course there is always Mr. Universe. He likely uses 2-3 of these cards.