Thank you for the very interesting comments.
I stand corrected on political art being mostly on the left. I object to ALL political art.
My claim is not that political art is socially worthless. Rather, it is this: political art cannot count as evidence for the political position it tries to advance. I happen to believe that political positions should be supported by argument, not by force, deceit, or emotion. (Some have blasted me for this, but it is my view...) A work of art is not an argument.
And the issue is not merely theoretical. I have seen many young people in my own native country, Argentina, join political violence, their revolutionary conviction greatly strengthened and fueled by protest songs and other politically committed art. They were not only wrong on the merits. They are, sadly, dead.