You could plug anything that's low impedence and balanced into them and it would work fine. You just can't plug things like guitars or keyboards into it (way to much wire to go through).
my studio snake and wallbox 1/4" carry headphone feeds back into the live room. i have a stereo feed that goes from control to live, and a samson s-phone in the live room to distribute the headphone feeds.
i now have those feeds linked to my Mackie Big Knob (hate that darn name!) monitor controller and so i can send two or three different mixes back to the live room.