It is a mistake to say a documentary has value if it challenges the idea of "documentary." There is no value in challenging the epistemological foundations of documentary; that challenge was begun prior to the invention of moving film and completed (at the latest) forty years ago. Instead the most we can do is fuck around with the shards of categories thought (perhaps nostalgically) to have been at one time not so long ago stable and meaningful.
We must pretend for a moment that we believe in the observational mode of documentary, with its long takes and synchronous sound which seem to form an exhaustive depiction of an exemplary everyday which is both banal or normal (to the subject/s) and extraordinary or abnormal or at least of interest (to the audience). In observational documentary there is always an other. (No one ever comes to observe themselves: those are home movies, whose veracity fools not even grandma.) It is in this pretending, this movement from knowing to not-knowing, that a form of dramatic play exists which can be more exhilarating and seductive than fictional narrative; a wholly different kind of engagement is required.