Jack

I am not a trained musician, and I'm not an instrumentalist, but I'm going to stick my nose in anyway ...

You DID ask, "Has anybody every done a scientific study of whether harmonization makes a difference to typical folk material?" You might like to take a look at "This is Your Brain On Music" by Daniel J. Levitin. (Available through the Mudcat Amazon link!) It doesn't deal with 'folk' per se but you may find it useful.   

It's also worth keeping in mind that, in some traditions, harmony is a key element of folk music and commonalities of harmony are part of the musical social compact entered into between musicians. Changes and developments to the common denominators are 'negotiated' and based on trust and respect. It's why group music making, particularly in the 'folk' genre, is so important to many broader social issues in my work (community social and economic development). I'm currently reading, "Music as Social Life: The Politics of Participation" by Thomas Turino which is helping me quite a bit in thinking these things through and giving me bullets to shoot at funding agencies!

Re: obvious harmonization as an important feature, there is a very large and important sub-genre of Welsh traditional music called "Cerdd Dant" (Poetry/song of the Strings) or "Canu Penillion" (Singing Verses) which is based on tunes, harmonies and counter-melodies. It is, at its most basic, a harp playing one melody and the singer holding forth with his or her personal selection of verses that fit and singing them to a counter melody which starts after the instrumental melody, avoids landing on the same notes, but ends at the same time - with the metre of the verses complimenting the metre of the tune. Sort of. There are a few English language pages on the Cerdd Dant
Society's website over on the upper right hand side. I don't think the 'History' article gives sufficient info on the counter melody bit, but you might find it interesting.

The point is that the quality of a cerdd dant performance will be completely assessed on its harmony and assonance, both of music and words. There's a whole festival in November just for Cerdd Dant and, trust me, the people who go to and / or compete at it KNOW what to listen for. Can be very scary ...

sian