The point surely isn't that the DT entry and it's notes should say everything there is to say about a song, and supply all the variants, but rather that any information that is given should be accurate, and tied down to sources.

Rumours have their useful place in the process of chasing down the facts - very often someone puts up a rumour or a speculation, and that prompts someone else to come up with the facts that lie behind it, of which maybe contradict it. That can sometimes happen a long time later.

But there a distinction needs to be drawn in DT notes (or whatever) between claims that have some solid backing (it may still be speculative, but speculation with a basis), and stuff without any real substance. (Which isn't to say they mightn't be true.) And then there are stories that persist but have been proved to be false. (Such as the story of the Fields of Athenry really having been published first in the 19th century.)