"But surely every performance one gives of anything should be to the best of one's ability which surely = using such 'expertise' as one has... "

Yes, definitely. I'm all for personal interpretation and brilliant performances. But nonetheless I can remember performances during which I felt that I was being treated primarily to an exhibition of vocal or instrumental technique, and that the performer had been less interested in the ballad and its story than in their own accomplishments. Where you draw the line is of course a matter of personal taste.