As the Sandman pointed out, the notion that a collector should include every item a singer performs isn't new. For instance, D. K. Wilgus criticized Cecil Sharp for omitting from his Appalachian collection recently-composed parlour songs known to be popular with mountain singers at the time.

Steve Roud's scholarship certainly isn't shoddy, and on the question of commercial songs becoming absorbed into 'folk' repertoire he's pretty much in line with the 1954 definition, stating at one point that a song ideally needs to have been passed down a couple of generations to qualify. Though, as I discussed in my post of September 29th, he does leave some ambiguity about that.

Sam Larner's Sankey & Moody hymns are an interesting case. Going back to Sharp, one of the criticisms levelled persistently at his work by American scholars is that he ignored the hymns that were a vibrant element in the repertoires of many Appalachian singers. This isn't actually correct, since he actually noted down several hymns that were straight out of books like 'Southern Harmony' and 'The Social Harp', knowingly or otherwise. All of these have Roud numbers - how could it be otherwise, given that they are in Sharp's collection? So should the Roud number be withdrawn because a given hymn is shown later to have come from a book? I've no doubt that Steve has thought carefully before deciding whether to allocate a number, for every one of the songs he's examined.

Anybody who has ever tried to sing what we have always accepted as a folk song knows that they differ greatly from all other kind of song

A lot of the differences that you and I would perceive are matters of musical and lyrical form, which in turn relate to the historical period in which many of our classic folk songs evolved. I understand that there are some (including previous posters to this thread) who have preferred Roud's FSE to talk more about the songs themselves, but that isn't the point of the book. It's a very thorough historical analysis of vernacular singing, that treats the Sharpian concept of folksong with respect but doesn't confine itself to that concept, and thus encourages us to at least think about its strengths and weaknesses.