(Someone tell me if they hate thread bumps!)
I just stumbled onto this, and found a great use case for it.
So ya' got's this file, see? And it's called Project Update December 2011 with annotations.pdf, which is a scan file. But then you add something else, and scan it again. I don't throw out partial work, I save it as drafts. But now it's no longer good! So I'm stuck scanning the new copy. But computers won't let you copy a new file with the same filename!
So this is neat so that when you rescan it, and it comes out Scan265432, you Switcheroo it, your fresh new scan gets the right file name, then you can rename the junk one Project-Junk etc.