Another wonderful thing about Sony's "Music" app is its ability to delete certain file from mass storage device. It is available there when viewing playlist (NOT editing it NOR viewing Media Library). Not so intuitive...
I would welcome to have such option in foobar mobile. But it should be present in ML view in first instance - as long-press menu command. Then when long pressing on file during editing playlist (adding files) - editing in the way described in my topic about redesign of playlist creation/editing scheme.