Putting Stevens' inability to access or understand Precious back on Stevens' race and class is a lose-lose copout.
Art is an empathy delivery system: its finest, highest purpose is to take situations like Precious' and make them available to others, people who have never been raped by their father, people who have never been pregnant with their father's child, or abused so terribly by their mother, who are not black, and so on. Art has been making horrific, grueling, grinding stories relatable for a long time now. That's its power.
According to your argument, the directors made a purposely bad movie that can only speak to the choir in order to perpetrate a division. Either you get the dehumanization because you've lived that specific brand of poverty or you don't and are never going to. That's one way to make art, I guess, but it's not a particularly powerful way to make art. Art is so good at building bridges of empathy between people who have vastly different experiences. Why stop at eliciting reflexive guilt out of some middle-class white folks and leaving others (the savvier ones who are probably the best possible audience if the goal is to really start understanding one another) scratching their heads? Rubbing white people's noses in racism is predictably satisfying. But why stop there, with this serious subject matter, when so much more is possible?
Racism and poverty hurt way too many people to play infantile power games with it. And the list of black artists who know this and have reached out with long arms, generating substantial, powerful, healing empathy is too long for me to buy your argument that Precious' failure to reach Stevens is Stevens' fault.