Either/Or Logic and Documentary
If I might add my own split-the-difference argument, I would note that the discourse on documentary often indulges in either/or logic. Either documentary is reality or it is a construction duping spectators. (Thankfully some good scholarship has thought through this issue more complexly.) Either we look to documentary traditions or we sweep the traditions under the rug to make way for brand new nonfiction forms. Either documentary works according to a Griersonian ideal of a public sphere instrument or it fails.
On this last point is the ethics critique that Brian Winston poses - though others like Calvin Pryluck have made similar critiques - namely, that the use and abuse documentary makes of the social actor is not justified by the ends of public sphere debate:
[T]he tradition of the victim inevitably requires that some measure or other of personal misery and distress be, if not exploited, then at least exposed. The justification for such exposure is the public's right to know as a species of public good. Yet if no, or little, social effect can be demonstrated, how can that justification stand? (Claiming the Real, 46)
A good point, especially as it suggests that public sphere institutions have their own professional ideologies and self-interest, which present themselves as universal interests. Still, do we have to decide between documentaries instrumentally changing public policy or social/economic conditions and documentaries having no effect? In fairness, Winston does give a condition - if no or little effect can be demonstrated - but his tone here and elsewhere suggests he does not see the effect in practice, at least according to traditional models of the public sphere.
Yet just as some sociological research or some journalism more valuable than others, some documentaries do a better job at serving as public good, which may include its value as a public sphere instrument. And the public sphere utility cannot be measured simply by results; representing an unrepresented position in the public debate adds qualitatively to that debate, regardless of the outcome. Much as in social science research, there are enough documentaries which do not have much impact public sphere that we should be cautious or humble in applying Griersonian aspirations, but I don't know that we need to reject them outright.