Not being 'bright' enough to 'do' tunes I regard myself as a 'wordsmith'. I always work with a 'trad' tune in my head, thw words are then put out to musicians. Some accompanied, some unaccompanied, what to use, guitar, fiddle, concertina, or just voice surley depends on the song and the performer[s], I don't think you can be dogmatic on this subject. All I will say, as I said to WAV some time ago, is the traditional instrument for traditional English folk song is the voice!