adding MPLS without scanning doesn't process disc library information
Disc library information includes things such as a disc's title and cover images. However, there can be more than one disc information file on the disc. Therefore the user has to chose which one to use (the GUI cannot auto-detect this).
Chosing is only implemented via the "select playlist to add" dialog at the moment, and that one only comes up when the user opts to let the GUI scan for playlists.