I agree with that opinion. The point of the ballad seller singing the song on the street in order to sell his wares was that by and large he was singing it to a fresh audience who hadn't heard the song before so they would be none the wiser regarding the tune. At the same time familiarity with an aspect of the ballad is often desirable from a buyer's point of view, and using a well-worn tune could supply that want.

I'm aware of thread drift here but it did follow a natural progression.