I think we need to separate diction (what Dick is talking about) from accent/dialect (what Richard is talking about).

It was, when they were alive, possible to go to hear Willie Scott or Davy Stewart, to take a couple of examples, and to be deeply puzzled by what they were singing. That is a clear case of what Richard calls a shortcoming in the listener. Performers like this had no diction problem - they were singing naturally and were wholly comprehensible to their natural audience, and if the sassenach wanted to listen in that was their problem. Certainly none of them were being promoted heavily by the music industry to force themselves on unsuspecting southern audiences.

Bad diction on the other hand, consists of wilfully adopting styles or failing to learn basic techniques or (in my experience this is most frequent) overuse/poor use of amplification in such a way as to make themselves incomprehensible to their intended audience.