It could be a useful item, on occasion.
I think the idea of copying image files is the "safest" approach - on the principle that you should always avoid messing about with the original. ("Don't burn your bridges...")
Same principle seems to be applied in Google Picasa3 - you can make changes (e.g., crop it, sharpen it, etc.) to an image for display purposes, but they are reversible. But you can make it "permanent". This seems to work so that, if you want to save the changed image, then Picasa will by default automatically back up the original to an "Originals" sub-folder in the directory where the original was located. The saved (modified) image is saved as the same filename as the original was.