Flying with the seagulls in the clouds.
One major goal of Kovid's work (perhaps *the* major goal) is to permit calibre's database not to be on a disc on the LAN. If he is successful (and it appears he will be), then there is no guarantee that *any* files in the library are available locally. No cover, no formats, no folders for authors/tities, no metadata.db, etc.
Of course, most people will have their library on local storage. Problem is that you can't count on that.