Thank you very much for quick response.
In fact, I was trying to remove empty dirs, dirs with only jpg / opf in my lib to make a cleen library (and yes, without my empty entries.
But it was a combination of testing the option and cleaning. I've other tools to clean this for me.
But thank you for making clear why this is (in other cases) a bad thing to do!
(And great you have build in a function to check this behavior before acting!)