The examples brought up most of the time to defend political film making tend to be focused on specific, concrete injustices too. "The Thin Blue Line" almost always comes up as a prime example. I don't think any recent politically motiated films--and there have been some great ones--have done much good at all.
Except maybe "An Inconvenient Truth". But I think the film itself served more as a lightning rod for media attention with its provoking right-wing commentators to admit the validity of global warming.

Everyone with a TV can watch Frontline and become more aware of the world for free, and they don't bother to. So how can we expect movies to do anything?