Well in the case of Oh No not the Bloody Fields of Athenry, and the parody[ies] of Waltzing Matilda, they are just a way of saying "we've heard enough of the original song thanks".
The Wild Rover was an early victim of this with just a few silly word changes like 'pulled out something white' instead of 'pulled out sovereigns bright' etc
As you say George a well written parody is a thing of beauty whereas some of the crap that gets trotted out in an effort to amuse the cognoscenti should be classified as a capital offence.
I do agree though that if you don't know the original most parodies are lost on you.
I remember someone getting upset at Sidmouth, and on that occasion rightly so, when some members of the audience insisted on singing the silly words to the chorus of the song she was singing, while she sang the correct version.
So it is an all or nothing activity I think.
Giok