our indigenous culture seems to have become so degenerate that most people equate 'music' with the written stave and think anything but manuscript music is not really music- see how jazz (especially the traditional variety)is looked down on. It's a widespread, if mistaken, mindset and who can blame folk seeking to take part in an enjoyable social experience for starting off using the dots/ written lyrics. Some people have an innate facility to remember tunes/words but some folk don't have that gift, and surely shouldn't be pilloried for 'cribs' even after years of involvement- that is if we are the sensitive folk we think we are. BUT it is not really acceptable if money is being charged- this is a 'performance' and the 'performer' certainly has a duty to give a paying audience value for his/her fiver or whatever - forgetting words is one thing, and as we (me!) get older it happens more often. I know of some older and well known performers who happily use a 'crib' and audiences accept this- they've earned respect over the years, and the alternative is to stop performing altogether...one such singer told me that since he's accepted the cribsheet/music stand as a necessity, he's never needed it!
Am not a believer in learning from printed sources, but some 'traditional' singers of past years certainly were! Maybe musicians are different- I loved a quote Joe Burke made once about a tune he loved dearly- 'I knew this tune before I learned it'- doesn't that speak volumes?