That's the rub: political disagreements tend to go beyond facts and into the realm of values, at which point it's no longer a matter of informedness. This phase change in the debate can be hard to detect, as even the debaters might not realize they are no longer arguing from facts (since their deeply held values may 'feel like facts'), but IMO it's the reason political discussions tend to decay in quality at a certain point, because one side is no longer honoring the previous terms of the debate, knowingly or not.
For what it's worth, I think political debates on HN are worth having and can be very productive, because my subjective sense of the HN population is that a higher percentage of us are open to reevaluating our beliefs (and potentially our values) in the face of new information than the human population at large.