Headphones as microphone

I remember doing that a looooong time ago when cd's were first coming out, and my tape walkman had a record feature on it. But I was a kid having fun haha.
 
I've never done this to record an acoustic guitar, but it works extremely well for tuning an acoustic that doesn't have any pickups...just plug right into the input of a tuner.
 
I have an old pair that I use to "program" drums with on occasion. Plug in and start tapping away. Works just like a trigger track.
 
I even saw someone using this on a guitar cab for high gain sounds.

EDIT: Only in addition to a SM57 of course