It would be nice to have a simple way to give an application exclusively rights to play audio. For example with a button in the applications view of the volume control. The problem is, that if you play music in a public event, you don't want to have any sounds played except for the single application you are using for playing music. The workaround I use for this case is to set some dummy output device as default output and only set the application to the real audio output. This works fine, so this feature is just a nice-to-have.
This is a reasonable use case, and one anyone who has to do a presentation will appreciate.