If someone has learnt a song but suffers nerves or is easily put off when others join in I don't see that a nearby crib sheet (in case of mental melt down and blind panic) should be a problem.

This is quite different from someone singing from a book or whatever, having made no effort to learn the song in the first place.

Having said that, not everyone can remember song lyrics effectively and I don't see why, if a performance is good, why the performer should be condemned because they had to read the words. What happened to tolerance?

What's the difference between reading words and reading a musical score (apart possibly from the degree of difficulty in remembering the latter)?