I think improvisation is essential to keeping traditional music alive. Very few players play a tune precisely as it is written down, because the notation in itself is someone's interpretation. There is no right or wrong for a tune - such is the folk process.

Dick, I'm sure you play tunes (as I know I do) with nuances which differ from someone else's interpretation of the same tune. It has a lot to do with personal playing style.

A classical melody or 'set piece' should not be changed, because we know that is the way the composer intended it to be played. We don't have that restriction in traditional music. We have an indication of the way people have played it in the past, which isn't necessarily 'correct'.

Improvisation in the jazz sense is different from inventing melodic or chordal variations, but there's no reason why it shouldn't be done that way - Blowzabella and Bellowhead are just two examples.