The use of "Show all tags" is questionable when dealing only with audio files because MC displays a long list of fields related to the Image view and the Video view as well as the Audio view.
If I put a value in a tag (and I'm assuming that by "tag" one means "field") I expect MC would display that tag/field and its value when I select "Show tags with values". All of my custom fields are displayed in the Tag Action pane.
MC displays "Bitrate", "Compression", "Playlists", "Rating", and "Sample Rate" in the Tag Actions pane even though I have not populated these fields nor have I placed them into my Audio view. And when I select "Show tags with values", MC displays "Bookmark", "Date First Rated", "Date Created", "Dynamic Range", "Peak Level...", and "Volume Level" as well. Those fields are populated (but not by me) but not displayed in my Audio view.
As a matter of fact, the "Duration" field is never displayed regardless of what I select in the Tag Status pane. I suspect that this problem is something that was lost and forgotten in some previous version and if so, maybe it could be reinserted in the next bug release??