> But I would much prefer a "An application is a directory and by default cannot access anything outside of that directory" approach.

That makes sense if the application is the only program that needs to interact with the data. For example: If you have a drawing or photo editing program. You might have downloaded an image from the internet or from your camera. Then you make some edits. Afterwards you want to send the image to someone else via e-mail, which is another program.