If I edit a png file using gimp (open file and make some changes) and then save, I expect that gimp finally make changes to that png file. Whether it stores session or layers or whatever in xcf should be transparent to me.
I don't need to separately export it again to png. Because... I opened a png, I made changes, saved it and closed the file.
If gimp not doing it, then it's an issue.
Maybe, don't let people to open other formats. But let them "import" other formats. This will maintain consistency . Whether usability improves is another question.
The difference is that with "save", the user should be able to expect that all (or at least the most important) aspects of his image editing session are retained. That includes, for instance, all the layers, but could perhaps even extend to the undo-history. It's up to the developers to draw the boundary, I guess.
With "export" on the other hand, such guarentees are not given. If I export my Gimp image to ".jpg" then I won't be able to restore the layer information later on.
Thus, there might be situations where I want to "save a copy as" and also situations where I want to "export a copy as". But they are different things.
Now, all of the above are theoretical considerations. It's not hard to imagine that developers accidentally, or even intentionally, divert from that distinction for whatever reason. But I think it's potentially a useful decision to make, because when you "export", that comes with an implict warning that what ever is written to disk might potentially not retain all information you might want when you wish to continue your editing session later on.