“(1) How can students be well educated when they only hear one side of the argument? … (2) Do universities need affirmative action for conservative professors?”
Two OpEds.
If a professor is competent enough, they won’t have to reveal their political ideologies to their students, even, or maybe especially, in a Political Science course. I’ve had a number of professors who were quite liberal (or conservative) in their private lives, but once in the realm of the classroom seminar, they functioned as the objective moderator.
I echo what was said above.
Also, I think it would pretty darn hypocritical for conservatives to demand affirmative action when they’ve been working to dismantle it for decades. After all, we live in a strict meritocracy where the most qualified person is always hired. If a conservative is the most qualified then s/he will clearly be hired 
And to chime in further, why the assumption that if someone’s liberal they can’t present themselves as neutral? This is particularly disturbing given that some (not all, though!) of the op-eds one reads calling for more conservative faculty state, usually implicitly but sometimes explicitly, that conservatives would be better able to position themselves as politically neutral in their teaching.
Second: In the vast majority of academic subjects, the political leanings of the instructor make no difference whatsoever. I mean, I spend weeks every semester teaching my students the features that go with various speech sounds. I really do have no idea how it would make any difference whether I’m a liberal or a conservative (or something else) in such a course. So I teach my students that a [v] is a voiced labiodental fricative—just as someone who disagrees with me in every way politically would. So why is this even a worry, at least in most cases?
Part of the issue is that the more educated a person is, studies have shown, correctly or not, the more liberal that person is. According to this line of thought, professors are highly educated and therefore more are liberal. The hiring pool is more liberal.
Correlation is not causation, except maybe sometimes it is.
Does education cause liberal ideas? Or maybe liberal people are naturally smarter on average and pursue higher education? Or maybe smart conservatives pursue nonacademic fields such as business, military studies (which are really creative and difficult), and other fields that are traditionally thought of as leaning conservative.
http://www.americanscientist.org/science/pub/study-are-liberals-smarter-than—conservatives
I think part of it, at least, is that higher education in many fields requires an openness to some ideas that might be distasteful in some conservative circles. A lot of the evidence discovered in the social sciences in the last century or so challenges conservative viewpoints pretty strongly - and, conversely, some conservative viewpoints fly directly in the face of modern science. (But then so do some liberal viewpoints - most of the anti-vaccination, pro-organic, and anti-GMO people I know are liberal.)
It also assumes that liberal professors are incapable of teaching both “sides” of whatever, or in recognizing the weaknesses of their own “side”. I’m pretty liberal, but I love hearing a well-considered argument between opposing sides in my classroom and in debates and discussions I always have some students argue from one side and some students argue from another. I often assign them viewpoints to make sure that both sides are adequately covered (and that my students know how to argue for something they don’t personally believe in, a useful skill). I rarely disclose what my personal political feelings are about something. It’s really not necessary or relevant in most cases.
I wouldn’t have a problem with ‘affirmative action’ for conservative scholars, though. It would certainly make graduate school and academia a more interesting place to be. I think I’ve mentioned before that one of my favorite high school teachers was a very conservative American government and economics teacher. He challenged a lot of the assumptions and frankly faulty or holey knowledge I’d picked up along the way - for example, no teacher had ever turned on the light bulb that money for free and reduced lunches have to come from somewhere (aka your paycheck!)