Comment 6 for bug 1514773

Crates can only hold one instance of a track, whereas a track can occur multiple times in a Playlist.
Adding a track via checkbox seems reasonable but what should happen when a Playlist is unchecked? Removing tracks might screw up the concept of a Playlist, so I'd say Playlist management (play order) should be done with the playlist visible in the main tracks table.

Don't know if that's the reason why it's not implemented though.