well I think you've hit the nail on the head. For academics with an encyclapaedic knowledge of folk cultures of distant lands and a desire to tabulate the results of his musings. well then definitions of what a ballad is, must be bloody nearly essential.

For the local halfwit with a headful of predjudices and needing through bitter resentment to discount the talents of more hadworking musicians than himself, and frantically searching for reasons to nullify their careers. Definitions are also pretty useful.

Sad to say that second use is the one you see most commonly on the English folkscene.