The situation I had in mind was one where a singer comes into a session where it's usual for songs to be accompanied by all sorts of instruments (which means zero to three sharps is the territory in common). Insisting on E flat is just silly in that situation, no folk song covers such a wide range that D would be impossible.

With singers who don't bring their own guitars, I've seen it happen that they insist on an E flat to start, somebody gives them a D instead, and they never notice.