Steve
As you say, one of the problems with discussions like this is that people tend to treat Ewan and Bert as coming from the same direction - they didn't.
Bert was a scholar (!) who sang, Ewan was a singer whose whole aim in life was to help create a situation where folk-song was accepted as a serious creative art.
Most singers I met (including me, when I sang seriously) adapted songs to suit themselves, their personal tastes and their circumstances - often described as being "part of the tradition" - I don't believe it was, but that way be dragons!
Ewan did it extensively throughout his involvement with folksong and if you tackled him, he would admit it and explain what he did (when he could remember).
Some of his discoveries/creations, like Alan Tyne of Harrow, became standards in the repertoire - certainly in mine, and I certainly wasn't bothered which was his and which belonged to the tradition, not as a singer or devotee of song anyway.
If Ewan had any claim to scholarship, it was in the work he did on developing singers, his work on relaxation, on analysing songs to make them part of the singer, his introducing Laben and Stanislavski into singing work - that was MacColl's genius.
Unfortunately, we have never got round to discussing this part of his contribution because of the garbage-mountain that miraculously appears whenever his name is mentioned.   
Ewan wasn't someone who you would automatically think of if you wished to learn of song origins - he was a singer's singer.
Bert was very much a different creature; the few times I managed to talk to him I was left with the impression of somebody who couldn't quite make up his mind what he was, singer or scholar.
I have to admit, I never found him particularly approachable or generous with ideas or information, not in the way I found Ewan and still find Peggy.
I found him remarkably entertaining to listen to, not so much as a singer, but certainly as a speaker (maybe 'talker' is a better word) - his 'Folk Song Virtuoso', 'Songs of the People', and 'The Lament' still have the power to lift the hairs on the back of the neck, after dozens of listenings.
Not sure how academically reliable they are, I suspect somewhat flawed, but certainly inspirational, which was exactly what I and many like me needed when we first heard them.
Both Ewan and Bert played an important part in my enjoyment, and later, understanding of folk song - that neither of then were able to walk on water was immaterial, and it would have been unfair to have expected them to do so.
If Ewan and Bert had anything in common, it was almost certainly their ability to inspire and encourage people to lift the corner of folk-song to see what was underneath.
For that I will be eternally grateful.
Jim Carroll