There's a concrete scenario I'm missing with rhythmbox (and also all other popular audio players I know). I've got both music and non music (eg audio books) resources to play with rhythmbox. Therefore I've created 2 folders on my harddrive called Music and Audiobooks. When I import the folders all resources will be added to one global library. So imagine I'm using rb as my music player at a party - how do I tell rb to only play music even if shuffle functionality is on or non experienced people are clicking around?
AFAIK the only way to separate them in rhythmbox is by adding an automatic playlists that filters the paths of the folders. The drawback when using playlists is the missing of the browser views above the files. Also lesser experienced rhythmbox users may overlook the playlist feature and use only the library functionality which contains both music and audiobook files.
So it would be great to enable or disable whole folders for playback in rhythmbox for example by allowing multiple audio sources from multiple folders within the library and let the user select or deselect them.