Taking the question generally, aside from the specifics of the discussion in the RB forum -- and it's a wonderful question, carbro!!!! -- I'm one of the "there's a platonic ballet out there somewhere" school, and that's what I want to see in a performance. I want to see the ballet. NOT the ballet as it was in 1841 or 1946, since I don't know what it looked like then, but a performance that lets me feel that I've seen the ballet. So I would say a masterpiece is a masterpiece. You may not like it, you may not be seeing the greatest performance of the ballet ever danced, but hey, that's not the ballet's fault.
In the "I like what I see before me" versus "It is NOT as it was with the first cast" discussions that we've had for six years on this board, I'd say they almost always break down to: those who saw the first cast vote for the "It's NOT the same" side, and those who didn't see it with the first cast are quite happy to see what they're seeing, especially if they like the dancers. I've had these discussions with friends, too, and it's never failed to happen that eventually, one off the "First cast phooey they're better now!" people will see a ballet that they had seen with its first cast become something else when danced by others and come round to the "It's just not the same as" side... at least with that one ballet! ("consummate performer in the role" can also be substituted for "first cast", of course. I know people who saw Galina Ulanova's Juliet and that's that. (I never saw it live, but I don't doubt them.)
Balanchine, clever fellow that he was, gave his ballets away to so many companies that alternate versions of the truth abound. As long as the steps are there and the structure is there -- and his scaffolding is made of the same material as the stone from which Arthur pulled the sword -- and at least a recognizable version of the Balanchine style is there, you see the ballet.
Ashton, who really truly wanted his ballets never to be danced by anyone except his first cast except, of course, he wanted his ballets to live
, is harder. I wrote on the RB discussion that I got a jolt the first time I saw different bodies, and especially different feet, in Ashton's "A Month in the Country" (one of a very few Ashton ballets I saw from their first casts) because Ashton is such a textural choreographer -- one of the "ballet is a moving painting" school -- that if you fiddle with proportions, you're wrecking the painting. Yet you have to "wreck it" for the ballet to live, and in ballets like "La Fille Mal Gardee," which have been danced by many different bodies, one can still enjoy the ballet.
There's a good book on this -- Selma Jeanne Cohen's "Next Week, Swan Lake," which gets at the "what is the work?" issue. (Concept, steps, score, story, style.)