"The film has some wonderful cinematic moments and some great work by its lead actors, most notably Guy Pearce who gets through most of the film with barely a word of dialogue."

Pearce was good and I'd say that the lack of dialogue had something to do with it. I thought the screenplay overall was rather weak. The peripheral presence of morality contradicted the characters' actions; Cave would've better off eschewing it altogether in this "armpit of the world." While you're right about the characters being symbolistic (after all, The Proposition, like any other recent Western, is "revisionist") they also felt hollow and uninteresting. I was curious about Arthur because the film tried to build him up in the early going, but the result was a miscast Danny Huston and his empty laugh; Winstone might've been better suited for that role. 2005's The Three Burials of Melquiades Estrada had an inherintly flawed premise, but it was well-executed by Jones. I'd say that the opposite is true for Hillcoat's film.