Of course some singers are able to put across a good performance while singing from a book. In my experience it is unusual for good singers to need a book, and there are usually special circumstances why it's being used. And if it's still a good performance, the book doesn't matter.

However, let's be honest, in the majority of cases the people using books are not good performers. All too often the book doesn't actually help their performance - they may not forget their words (assuming they don't lose their place) but by concentrating on reading the lyrics they don't pay sufficient attention to other aspects of their performance.

If they're going to become better performers they need to focus more on their presentation and delivery, which means they should be solid on the words (and chords if they're accompanying themselves). If they continue to rely on the book as a crutch they'll continue to turn out poor performances, which is no good for the audience and probably no good for them.