[Steve in Pudsey]

All stations will have some backup source but I can't see any need for them to run permanently to dip in and out of rather than starting as required. Some smaller community/student stations have an automated Jukebox overnight which runs constantly, but I think using it as a back up in anything other than dire circumstances is hugely frowned upon.

The only other example I can think of is in TV, the BBC Nations have the breakdown slide permanently available as a source with music (at one time the idents soundtracks on BBC One) so the lone Director/Announcer can get it on air instantly and get on with dealing with the fault.