Well, I've never seen Davy Graham, but I would not have been encouraged to start playing the guitar if it was not for him, so I hold him in high esteem . .
however, based on this thread, I won't be going to see him in the near future.
I started playing the guitar by learning Davy's Angi note-by-note from a Bert Jansch recording. I worshipped Bert Jansch & John Renbourn's guitar playing in the 70's and saw them both more than once in their heyday - they were brilliant. I then saw Bert again in the early eighties and he was absolutely awful - drunk on stage - I was so sorry for him personally and also disappointed that he had performed like that to a paying audience. I'm so very pleased to hear from "Grab" that Bert is now reformed and on form again. If he comes my way, I'll make the effort to see him.

In a somewhat similar fasion, I heard the classical guitarist Segovia when he was 86 and again when he was 87 - in both cases, he was mind-blowingly brilliant. But then I heard him a few years later at a concert in the Barbican Hall when he was around 93 and he was, let's say 'awful'. Still a lot better than me but playing loads of duff notes. I thought he would have been better advised to have stopped performing a few years earlier. Still, I'm sure that many people went to the concert just to say they had seen him and were content with what they heard, never mind the standard.

It may be the same for some performers in the folk, blues & rock sphere. I went to see Lonnie Donnegan just before he died and he was brilliant. But I've recently heard reports of other 'greats' who obviously have not fared so well.

Over the years, I've heard lot's of folk performers who have not been completely up to scratch on a specific occasion, even though I have known their capabilities on a 'good day'. Sometimes this has been due to a little drink or tiredness or age.

Preferably, I think a true professional should strive to give a spotless performance, but to expect perfection is probably unrealistic.

If we go to see old performers for nostalgic reasons, we should just expect to experience them as they are now, warts and all. Their contribution may well have been great at their peak, but just like an Olympic athlete, we can't expect them to still be contributing at that level forever. Also, for those who don't rate his 'seminal' albums, we could say the same about many things; many things seem insignificant in retrospect because we don't understand the world mind-set of the time they were created. How revolutionary was the idea of the wheel, the earth being round or electricity to a world that had not yet experienced these things ? To many, when first heard, Davy's playing seemed just as revolutionary.