Some people who play by ear are not at the point where thay can distinguish the difference between a good place to put a G or an Em7. Their musical ear ain't quite there. Also, they may not be able to find the sheet music. Once you recognize that a D13 flat 5 flat 9 doesn't provide the same 'background' to a melody as a D 13 flat 9, THEN the question would have validity. I would presume that someone would ask for chords because they don't have them. Good people will provide and not bitch about it.