I can't remember now what I needed this feature for: I just wanted to get rid of the albums of a particular folder.
I know now that filters can be used but it's not the same thing. If I don't want a certain folder to be part of my library, it should be possible to do this by simply removing the folder from the list.
Again: I can't think of a situation where I want to remove the folder from the list but keep all the albums in it in the library.
In any case, as I said before, nobody agrees with me, so that's it and I accept it.