Certainly one of the main ideas of good performance is to come across as if it is natural. The fact is, though,for almost everyone, getting on stage to sing and be engaging and make them laugh and cry and everything in between is not a natural situation. So..most performers practice enough songs to put together a show and sprinkle it with some clever patter to break things up. WHile repeating 2 shows for the same audience certainly demonstrates some degree of laziness, to expect a 100% fresh show is not realistic either. If I were to see a performer multiple times at a festival, I would assume he has booked several shows to reach more people, not the same people over and over...as such, I would give him a break. Also, if I like a performer enough to see them twice in 2 days, I probably would not much care what they sang. For the most part, when a performer finds something that works, they stick with it. For example, Josh White used to break the same string at the same point in the same song at every show. He would keep the song going while he changed strings - always to a big crowd reaction.