Remembering the words is part of the preparatory work for performing it in public. There are all sorts of 'tricks' for doing this while working on the song - singing it at double speed, taking the verses out of order.... If you can't do it, the song isn't ready to be aired.
Not being able to sing in tune - the same thing goes, but even more basic work needed.
I have said often enough that I believe anybody (almost) can sing, but if you are among the miniscule minority who can't, go and do something else. Otherwise you are like Monty Python's actor applying for the role of Tarzan.
I have spent abest part of a lifetime trying to dance, but I can't, so I don't.   
Pip,
Maybe the landfill sites will survive - no reason why they shouldn't, as long as they don't edge out any other type of music in order to do so.
When I lived in Manchester I often used to go to a pub on the Stretford Road where the landlord organised a 'singing night' - anything goes and a couple of free drinks and a fiver to the winner - wonderful. But on Fridays and Tuesdays I could go to Harry Boardman's and Terry Whelan's folk clubs and hear the best of folk songs sung well.
My objection is that nowadays my ability to choose my music has been removed by the 'folk club' organisers having torn the labels off.
"you owe that amount of respect to the music and to yourself."
Says it all really.
Jim Carroll