The only thing I'm wary of is The Rule, Michael - which isn't about to be proved by all these bally exceptions. And in urging a more descriptive approach to Folk Song, as oppose to a prescriptive one, then we might clarify the sort of thing that deserves to be classified as a Folk Song (Seeds of Love etc.) and those that obviously don't (Johnny B. Goode etc.), no matter how Traditional they might be otherwise (ICTM) as all musical idioms undoubtedly are.

Talking about babies and bathwater, I remember collecting this as a kid around the playground to a tune close to Glory, Glory, Hallelullia:

Whiter than the whitewash than the whitewash on the wall,
Whiter than the whitewash than the whitewash on the wall,
If you wash me in the water that you wash your dirty daughter,
I'll be whiter than the whitewash than the whitewash on the wall.

Ten years later I was singing it as part of Oh What a Lovely War (it's in the Metheun Script at least) and the producer was most fascinated to think how it got from the trenches of WW1 to the playgrounds of 1960s Northumberland.