To me, the salient point here is that NPR and PBS, while underwritten by foundations and businesses to sustain them, do not have a profit motive as their central goal. As an example, see NPR's mission statement here:
This affords both organizations a unique place in the American media universe, in that they have more freedom from the influence of advertisers in what they report and who they report on. An orgranization whose primary mission is to maximize shareholder value (most of 'em) might be hesitant to run a report or documentary that is critical of a potential advertiser for fear of its impact on revenues.
For example, Frontline did a report on Walmart several years ago that was not particularly flattering. It's hard for me to imagine one of the other news organizations running that same report and risk alienating a (possible) source of revenue as huge as Walmart.
I concur with the arguments that the gap couldn't be filled by other "cultural" channels. A news organization's primary mission will likely have an influence on the content they air. NPR and PBS are in a different game altogether than the commercial news organizations. All they have in common, in my opinion, is the medium.