I think some songs are meant to be sung communally - the repetition (perhaps with variations) being there to extend the communal feeling. So, at the end of a singaround, you might get something like "Mama Don't Allow", where various instruments get the chance to play together and the sung chorus allows everyone to join in without much need to remember different words.

(Interestingly enough, it's fun to sing, at some point in that song:

"Mama don't allow no silence in here - Mama don't allow no silence in here.
Who cares what Mama don't allow, let's have that silence any old how,
Mama don't allow no silence in here"

and then see what happens... all in the spirit of John Cage, of course).