[forgive me, as i don't know how to state this in concise technical terminology]
when you drag and drop to the media player classic gui or its playlist, or use the add files function of said playlist, mpc only checks the current directory for media, not the folder branches below it.
for those of us with organised hierarchical structures to our media folders this means we have to search out and individually add all leaf nodes of the tree structure in order for the contained media to be added to the playlist.
fi: if i have "band a - album a" with sub folders "disc 1" and "disc 2", the contents of those sub folders will not get added if you try to add the "band a - album a" folder to the playlist in any of the ways previously mentioned.
[aside from the necessary coding] an additional tick box option to "check all sub folders" would resolve this, in case some people wouldn't want it added as default behaviour.
i am suggesting this as additional practical playlist functionality, not (what i consider to be) an embellishment.
thanks for listening.