I guess if you can't imagine what to do with a larger desktop you don't need it. I've had three monitors for years, but had 2 computers doing it - including a KVM so I could dual monitor both or easily add a third computer to the mix to build or troubleshoot.
Most music creation suites love the room as does Photoshop, and video creation is much better with as much timeline as possible - I'm looking forward to a 5770 someday just for the third monitor for everyday work.
I'd rather get a 5850 or 5870 so I could seriously game with it - wraparound FPS and racing/flyin' games for me, but budget reasons keep me from goin' there I think.
If you are a graphic designer or a stock trader then its great to have more space to spread out your windows so you dont have to change between windows, just change which screen you are looking at. For gaming, with the new 5xxx series it gives you the option to do stuff like this