I started this post several hours ago, responding to one point of Jim's, but then got multiply sidetracked. So there will be more to say, but let's do this bit first.

Jim said
"Roud has cornered the market on the term 'folk song' - he is rightfully highly regarded for his work - now he has shot off at a tangent by lumping folk and non-folk material into this numbering system.
The Roud index has not suddenly changed its scope around the time of publication of the book. If it errs on the side of being too broad, including some songs with dubious claims to being "folk" songs, that's surely better than being too narrow and failing to assign numbers to songs that we may wish to investigate and discuss. Inclusion in the index indicates that a song is putatively a folk song according to some criteria but surely does not constitute a claim that "This is a folk song and anyone who disagrees is wrong".