Use of aide-memoires is intended to satisfy another requirement of etiquette: don't be repetitive. People who perform only from memory often have tiny repertoires and run out of willing listeners VERY fast. If you use a memory aid can keep it fresh, even playing weekly in the same venue.

This is largely a consequence of the mass media. The recording industry and its successors like Internet broadcasting can keep a supply of new material flowing far faster than anyone can listen to it, even within a single narrow genre. So if you're trying to do the same sort of thing in live performance, you have massive competition that wasn't there a lifetime ago. Nobody could memorize enough songs to compete with Tobar an Dualchais, let alone Sony or Spotify. To have even a shred of credibility as an entertainer, particularly when you also have a day job, you need that iPhone or you'll be boring.

I thought Walter Benjamin might have had something to say about this, so I just flipped through "The Work of Art in the Age of Mechanical Reproduction". He doesn't - he seems to assume that mass-produced art will just supersede the hand-made kind wherever they come in contact. Folkies just assume he was wrong without thinking about it. It might be helpful if they thought a bit more carefully about what our enterprise is trying to do in the present age.