Oh, I do think she's a lovely lady n'all and undoubtedly she has made an unrivalled contribution to folk - this is purely a "analytical" opinion of her voice. I do listen to Norma's voice though because it is unavoidable when listening to music for repertoire/pleasure/research, whatever...I think she has a great "voice" - but that's not the same as being able "sing" in the conventional sense of the word.
E.g. Annie Lennox is a technically a superb singer - i.e., pitch, timing, breathing, notes, etc., but some people I know hate her voice and so say she can't sing.
Why do people get so upset about this? It's purely an objective comment.