The file browser *can* already be set to only show Playlists or Music files.
I personally feel that the filetree should be shown as a "folder" beneath tagcache (which can be backed out of) but not the other way around. If tagcache is initialized the user may as well be in it if they want to use it, I think, but I'd like to keep the filetree free of entries that aren't really physical (this is just my opinion though).