Spaw,

It's a slippery slope. Ellington was a composer and a bandleader. To say he defines jazz in the 20th century doesn't quite make it because you have to ignore Louis (very important in the evolution of jazz either in combos or in big bands) and Bird and Monk who defined be-bop. This is Wynton's bias which may get him some grant money. Duke certainly contributed to 20th Century music in general but to say he defines jazz is not right. It would be more appropriate to call him a leading 20th Century composer.

To try to define folk in this way really creates a can of worms if you are trying to reach a large audience. It would have more problems than the Ken Burns "Jazz" flick had with all the musicians who didn't like it because it missed so much contemporary.

Try to do with with "folk" and you'll have all kinds of folks breathing down your neck. About ten years ago, the U.S. Senate had a bill before them to make Square Dance the national dance of the US. The Folk Arts Division of the Lib. of Congress thought this was a bad idea and finally I think it was tossed out.
You run into the same problems here. The last ten years or so, musicians in the folk field have resisted narrowing the definition that the academicians and folklorists had created and I think that this might be a good thing. It's confusing on one hand because when you ask the unitiated what they think is "folk" you'll get some surprising answers, many conflicting.

Frank Hamilton