That leads to the other paradox. Having commentators from the opposite side is not necessarily balance. Balance depends more on the neutrality and curiosity of the questioner. It is also more balanced to have moderates from either side than to have radicals from either side. Why?
Because an audience that has what they believe reaffirmed by one side will react against the other side. It tends to be more entertaining than informative. True. Ocasionally a listener might actually allow the opposition's side to penetrate the skull. But the ruler of the show gets first and last word.
NPR has a moderating effect on many of its shows because they tend to pick conservatives that know they are speaking to a moderate-to-liberal audience and generally avoid reactive questioning. Plus the first and final word are generally divided. And they tend to avoid appealing to "common sense".

But in all reality I think what makes balance impossible is the fact that people by and large do not actually want it.
Entertainment is a priority over thought-stimulation.

Because people are for the most part not actually effected by whatever biases exist in the media, I don't actually see it as a concern. What I do find disturbing is that the very roots of the contraversy are founded in the objection of the mode to being shown the sides of the non-mode. They do not want journalism that calls into question the things they take for granted or the perception of fairness in society. They want blissful ignorance.
And the left's reaction is more exaggerated, harder core liberalsim.
The country is falling apart because people prefer their own realities to the compromise between them. Conservatives want the news to tell them what they already take for granted but to hear an occasional radical so they can feel they are being well-informed.
Liberals want to break down what people already 'know" and they know it is an uphill battle so they overdo it.
I don't think there actually are very many moderates in the country. Moderates seem more like shifters and apaths for the most part. There doesn't seem to be anyone to hold the country together.

__________________
"Man lives in the sunlit world of that which he believes to be reality. But unseen by most is an underworld, a place that is just as real... but not as brightly lit... A DARK SIDE!"
-opening from Tales From the Darkside