Ballet companies are expanding their rep to include more and more modern pieces. This is fine, except that critics and fans nitpick them for falling short of the original dancers' style in the modern genre. Is this really fair? If the Merce Cunningham company decided to put on a traditional, romantic Giselle - wouldn't that seem strange? They wouldn't look like the Romantic visions because their bodies have trained in a different genre for so long.
I agree with much in your post--though sometimes it can help one to understand what one admires about one ballerina to compare her to someone one admires less--but the above seems to me an entirely different issue.
I don't think critics and fans are "nitpicking" when they see things danced in a way that often evacuates them of their meaning/impact and feel they need to say something. If a ballet company chooses to dance Cunningham, then I think it is fair for viewers to reflect on what those dancers are doing with it for good or ill. Someone might even say they like it danced by ballet dancers even if it is 'wrong'--but a knowledgeable viewer will know the difference and a less knowledgeable one who wants to know more may well be curious about what the difference is. And someone who has only seen ballet dancers dance modern dance classics (Graham, Taylor, Cunningham, even Tharp in some incarnations) may be forgiven if they sometimes wonder what the fuss is about.
Cunningham precisely did NOT have his dancers dance Giselle--modern dancers don't tend to pretend their technical prowess can take on anything and in any case most major modern dance companies were founded in part to do new work if not work in an entirely new idiom. Ballet dancers, who do have extraordinary prowess, are often capable of crossing over...up to a point (call it pun intended)...but not without costs that I don't think it would be nitpicking to mention.