That quote from "The Ballad Tree" makes no allowance for a ballad being adapted in transmission - the author seems to assume that they are "composed" once and for all. It seems pretty clear from the sort of scholarship Roud and Gardham have put in that that is almost never the case - they are not the property of the original author, or of the social class that author belonged to, and radical editing is the norm.

I haven't read Roud's book yet, but maybe he covers something that I haven't seen mentioned here. Many of the very oldest songs we know seem to be lyrical fragments extracted from much longer narratives, which may have been metrical or a mixture of prose and lyric. And the editing process of oral transmission often leads to what Evelyn Wells called the "fifth act" phenomenon - the sung jumps in at the point where most of the story has already taken place, and the best songs leave a lot implicit.

Which implies that the relationship between songs and patter is not accidental. If a song is an episode in a story that everybody knows, it doesn't take much explaining. If not, explaining the background is an essential part of the performance. And which bits are included in the song and which bits left to prose explanation is going to vary over time. You don't want to look at the evolution of songs in isolation - the meaningful unit is the song together with whatever in the performance and the audience's understanding makes its narrative intelligible.

Inclusion of too much story is what makes much of Scott's output unreadable today. He was trying to emulate the sort of written mediaeval tradition where every last pernickety detail had to be left in, since the songs were often about the nobility and they wouldn't tolerate being relegated to a footnote (for an absolutely appalling example of the original practice, look at "Graysteil"). Scott went in for the same sort of aristocratic arselicking that created heraldic emblems with every ancestor included inside quarterings upon quarterings. Whereas if you didn't have a noble patron to please, you could leave Lord Muck out or muddle him up with Prince Pigshit and nobody would care. Maybe the singer would know about facts that the song omitted, but there would be a tendency for them to get lost over time.

Does Roud talk about this sort of thing? If not, who does?