Jack, you asked
'What examples can anyone come up with where the harmonization is the distinctive and important feature of a particular arrangement of a folk tune - a particular harmonic effect that absolutely anyone would notice, and which made all the difference to how popular it became? How common are such examples?'

I think the main one that springs to mind is this:
House of the Rising Sun (Leadbelly)
compared to this:
The Animals

Having said that, you are probably correct that most of the intricate sus4 chords etc we guitarists put in are largely for our own satisfaction...
cheers
KP