At risk of appearing to contradict myself, I accept that crib sheets are sometimes necessary, even for professional performers. Sometimes you simply need a prompt for something which refuses to sink in, or sometimes you might get asked to perform something new (or long dropped from your repertoire) without sufficient time to rehearse properly. And not just singers - even professional actors invariably have a prompter in the wings.

For example, my band was booked to play for a dance, and we were asked to learn a number of tunes which were for specific dances. We didn't have much time, and they weren't in our usual style so we wouldn't be keeping them in the repertoire. We learned the tunes, but not as thoroughly as we normally would before playing them out, so we kept sheet music handy, just in case.

On another occasion I was asked to sing two specific songs during a gig. Again, I learned them specially for the occasion, but kept a prompt handy. In this case I had no opportunity to give the songs a "trial run" in a less formal setting, which I usually like to do with new material.

The important thing is, prompts should be used only when necessary, for prompting not reading off, and in such a way that the audience won't be aware of it - on the floor or against the monitor are good options.