Personally, I would far, far rather listen to someone sing a song with the words/music in front of them and make a good job of it, than suffer through someone's attempt to do so from memory if it has stops, slips and pauses in it!

I remember being very disappointed the first time I saw Steve Tilston in concert at a festival--He forgot words and just seemed very ill-prepared. Give me someone singing a good interpretation with confidence any day of the week, even if it takes an aide memoir.