Richard Mellish, I haven't done a census and can rely only on my own experience. I can think of only a few occasions where someone has successfully used a prompt. In by far the majority of cases I have come across the production of a book or song sheet is a bad sign.

The problem is not so much the presence of a book or prompt, this is only a symptom of a lack of preparation and a lack of confidence resulting in a weak delivery. Whilst a prompt may avoid the worry of forgetting the words, I feel strongly that you cannot develop the skills and confidence to perform to an audience if you have to rely on props such as these. Yes, it is difficult, and particularly at first it can be nerve-wracking, but that is all part of singing in public. Some Bloke put it better than I could when he referred to it as a courtesy to the audience.

Lead sheets have their place, particularly for those who may be called on to play from a very wide repertoire on little notice.
I have sympathy with those buskers and pub singers who may need lead sheets, and software now provides a convenient and discreet way of using these. However for an amateur singer in a folk club who is unlikely to be called upon for more than two or three pieces in an evening there is little excuse.