Masato's got a good question. If we take the plague-origin-theory not as a historical proposition, but as folklore itself, then we can forget about whether it's accurate (a substantial wing of folklorists regard origins as never completely recoverable, therefore uninteresting. I never quite followed the "therefore" part). So the question becomes, "what's the attraction of believing this?" or "what function does it serve for those who tell it?" As a first guess, I'd venture that it's an antidote to the romantic view of children to imagine them cheerfully singing songs about something so horrible -- it reminds us that they have to cope with some pretty hhorrible stuff.

Best,

Adam