So, although the argument against a particular casting choice might be frustrating on one level, on another, it makes complete sense. I believe the writer should always have the last word. Though there are two instances where I think non-traditional approaches can make sense
This isn't your grandfather's Les Miz. It's the new and enhanced-in-some-ways-diminished-in-others Les Miz, and it very much takes into account the movie and the possible expectations that it's planted in the eyes, ears and minds of ticket buyers.
Samuel Beckett wanted his plays done exactly as he wrote them and as he particularized them in his stage directions. Don't think of adding even a second tree, for instance, to the Waiting for Godot set.
We know that cats have nine lives. "Cats," based on T.S. Eliot's "Old Possum's Book of Practical Cats," with music by Andrew Lloyd Webber, with additional lyrics by Trevor Nunn, shows us how this process of reincarnation works.