he was perhaps as class-conscious as Woody Guthrie
Maybe, but he was not as political.
I don't know if you're calling Cash a "rock star" or if you just mean Cash's life during the period covered in the film is similar to that of a rock star.
One thing that struck me was how close to Elvis he was starting out, in physical proximity recording in the same studio, and it seemed like the genre lines were not as sharply drawn then as they are now. And since Cash was in a seense not that "musical" -- you mention, Oscar, his "minimalist" settings and limited baritone -- that, to me, puts him closer to a rock star and further from a good musiician. But let's not forget about the quality of the lyrics of Cash's song's. LIke Dylan, he's a vernacular poet with very evocative lines.

Oscar, you have a point about Jamie Foxx winining an Oscar wihout actually singing, it was an act of pure physical mimicry and dramatic acting, but a lot of people like the fact that both Joaquin Phonix and Ms. Witherspoon do their own singing; they feel it lends authenticity to the performance. I do too.