A fascinating thread, which makes many valuable points about two separate but related issues.   And there is a hidden link between the two themes - it's fear!

Many fine professional actors, singers, and comedians suffer terribly from pre-performance nerves. Some even throw up in the dressing room every night before going on stage. Not a few have been driven to drink, drugs or self-destruction by the pressure which the continual fear of failing puts on them. (The late great Tony Hancock was one famous example.)

A performer's worst nightmare is losing your hold on the audience. If that's the fear that haunts you, it's very tempting to stick with material that has worked for you many times before.   That could be the reason why some of the artists who've been criticised in this thread don't revise their acts more frequently.

The other great fear is of loosing your grip on your material. Memory lapses can hit the best prepared actors – that's why theatrical productions usually have a prompter on hand.   Likewise, some folk performers prefer to keep the full text of every song within reach, while many others have brief notes on cards up their sleeves, or taped to the side of their guitars. Why not ? Most drivers travel with a spare wheel on board, even though they hope not to need it.   

Even so, IMHO, performers should keep their eyes on the audience as much as possible, and only refer to books or notes in an emergency. Nervous beginners may feel that they'll never manage without these supports – just as many non-swimmers find it hard to believe that the water really will hold them up. But memory is like a muscle. If exercised regularly, it will get stronger.   

Wassail!