The stated goal was to make it harder (still? I guess people clicked through the warning without reading) to accidentally save in .png or .jpg and lose the layers. But the result, for someone whose usual workflow is to open a .png/.jpg, edit, and save back to .png/.jpg, could not be more user-hostile and obnoxious.
In response to a complaint, someone on the forums wrote:
> This is the way Gimp now works. Of course you can consider this a PITA. But for other folks, losing work because they saved as JPG instead of XCF is also a PITA. The philosophy is that people that are impacted negatively by this change are people with very simple work-flows who would be better off using simpler software (XnView, Digikam, Lighttable...)
In other words, all of you people with simple workflows, that worked just fine in Gimp for years? Now you have to go off and learn some new software, because Gimp is for Serious Graphic Design™ only. Ugh.
I got excited researching this because I found a changelog for Gimp 2.8.4 that mentions patching the behavior to be sane, but then I realized that's only a patch in the unofficial OS X build. On Linux, the same nonsense in the latest version.