Cap'n,
It is difficult to judge how good a collector Kennedy was because, as far I am aware, we have no access to his field tapes. He was fully financed and equipped by the BBC and was able to approach his singers with their authority behind him, which gave him an enormous advantage over most other collectors. It is often forgotten that he was a part of a team and was not working on his own, so the recordings he marketed were not just his own, but also the products of the work of others.
One of that team, Seamus Ennis, once summed it up for me in a conversation I had with him during a music session. I was unwise enough to mention Kennedy and the BBC project and Ennis became very angry and said, "that man is a thief".
For me, part of the judgement of a good collector is how he treats his informants - Kennedy doesn't fare too well in that respect. This is especially true when you remember that his victims included impoverished Traveller children from whom he lifted royalties aimed at their education - not exactly Woody Guthrie's "robbing pennies out of blind men's cups", but in that general direction.
I'm afraid I regarded buying Kennedy's material the same as I did buying South African goods during the Apartheid period - as useful as they might have been, I didn't!   
Incidentally, you are wrong about his being the only one to have recorded Neilly Boyle (I seem to remember Boyle was recorded commercially and appeared on 78s?) Kennedy recorded him along with another member of the BBC team, Sean O'Boyle, who carried out the interview.
You still haven't commented on your attitude to his behaviour towards both traditional singers and his fellow collectors. I look faorward to hearing it - but I won't hold my breath.
Jim Carroll
PS I left out the word 'recorded' in the first sentence of my last posting - please insert were appropriate (if you get my drift). Don't know where all those peculiar punctuations came from.