"Aren't orchestras (string quartets, etc) created to perform music specifically written for that purpose?"
The purpose of any music is generally to be performed. Just because someone writes their own music and performs it doesn't necessarily invalidate someone else performing it. I don't ever recall any of the Beatles saying nobody else should perform their music.
"Jarrett - Also writes his own stuff as well as interpreting other peoples does he not?"
He does. And your point is...? Why does composing your own material suddenly make it alright to cover other people, while not composing doesn't? Many of the greatest performers across all idioms of music never composed much, if anything. Many/Most classical superstars, instrumental and vocal, Sinatra, Aretha Franklin and so on. So is the Queen of Soul a talent free nonentity?
"However, some bunch of non-entities doing note for note covers, to my mind, doesn't add anything to the original material. A Gilmour solo is the real thing, a copy isn't."
Whether a note for note cover adds anything is irrelevant: that the audience themselves have a chance to add something is the point of live atmosphere. If someone, anyone, turns in a great performance of a piece I like and it lifts all the audience with it, so what if it wasn't the person that originally played it? And define "non-entity"... Think of Jimmy Page butchering Stairway to Heaven at Live Aid having recently consumed the entire Colombian drug export output for 6 months, and think if any number of anonymous session men wouldn't have done a better job in keeping the crowd along with a favourite classic. I prefer butter to substitutes, but not when the butter's rancid.
"It's like comparing G D Rosetti's 'Beata Beatrix'
with some copy; it can never have the power of the original."
So what? If it has positive power, that still puts you ahead, even if you're not as far ahead. You're effectively saying second best absolutely won't do. In which case why bother with your pathetic little stereo when it can never have the power of 3 500s, dbls, 52 and a cds2. Waste of time Steve, just pass it on to me...
Pete.