So that whenever I want to do exploratory work that I probably want to either fork or toss, I have to go into time machine to do it, rather than simply not saving, or saving under a new name.
Or choose Duplicate on the old file, don't save the copy/give it a name, and do the work in there.
Since Lion, OS X also internally autosaves files that have yet to be saved. I've had 3-4 "Untitled" TextEdit files that I haven't saved hanging around for months, across reboots.
I don't mind some internal backup-copy (including undo/redo state) being made of what's currently on the screen so that it can recover from a crash, but actually modifying the file I loaded from when I never told it to save is just bad, bad, bad.
Apple is quickly losing touch with reality.