Garrett finds it strange that the Times critic, Claudia La Rocco, hardly mentions the Fornes connection; he has some very thoughtful musings on whether it's a critic's responsibility to comment on an adaptation's relationship to the original. Mike thinks the reason La Rocco doesn't go there is that she doesn't have the knowledge; he points out that La Rocco is primarily a dance critic.
Oddly enough, I had a conversation on this subject at a party a few nights ago. (Asked about Frank Galati's adaptation of Kafka on the Shore, I said that it mostly made me want to pick up the book so I could figure out what the hell was going on. The reply, from a Murakami fan, was that it might not help.) I argued that I need not necessarily have an intimate knowledge of the work a play is based on, and that in the case of a thing like Kafka, most of the audience will be coming in fresh as well (putting me, I guess, on the "vox populi" side of Garrett's divide).
But had I been reviewing Kafka, I would absolutely have touched on the novel, Murakami, and Galati's history with the author. Whether you view criticism as art or a consumer service, part of a good critic's responsibilities is to provide context for the reader. Even if you can't track down a copy of a Fornes script before seeing the new version, 15 minutes online can give you some background to pass on to the reader.
I'd prefer to know the original in most cases, but if I were to recuse myself from reviewing a show unless I'd read the book seems not too far from saying I'm unqualified to review Chekhov unless I've read it in the original Russian. There's a certain point at which the work on view has to stand on its own.