Most of the previous comments are from the performers point of view, which I understand, but there's another perspective here too.

Festivals are great for discovering artists you've never seen (or even heard of) before. I've seen performances, liked the artist or a particular song so much I've gone back for a second time and then been disappointed not to hear at least some of the same material again.

Striking the balance is the answer IMHO; mix it up a bit but do the favourites every time, perhaps putting the variation into the chatter and anecdotes rather than the music.

Chris

(Not ther same Chris as last post)