Because a few doddering old fools such as myself suffer occaional memory lapses does not justify every bright young thing singing all their songs off their mobile phones. Of course people can do what they like, but those of us who against ring-binder culture are merely pointing out it doesn't generally leads to good performances.
The Coppers(in reference to an earlier post) did not actually sing off sheet music, by the way. Or at least they didnt when I saw them. They had the words, not the dots.