"It is a term that expresses some form of limitation on the part of the singer" is how the Shambkles sees it.

I'd be with Art on seeing it completely the other way round, a way of paying respect to the singer in question.

Any objection would be because there is an implication that other singers are a bit second-hand and second rate, and I'd be inclined to think that that would be what Norma Waterson would have had in mind.