Here is the superb statement of Professor John Ellis, president of the California Association of Scholars, to the joint committee on the master plan for higher education of the California legislature.

An excerpt:

My concern here [specifically with the leftist “monoculture” in political-science departments] has nothing to do with being fair to conservatives. . . . It’s about the dumbing down of education. One-sided departments can’t educate. John Stuart Mill put the point best when he said: “He who knows only his own side of the case, knows little of that.” What Mill is saying here is that you don’t really understand the case for the left until you also thoroughly grasp the case for the right, because the one is an answer to the other and so each is a necessary part of understanding the other. If leftist professors think they can simply present the other side’s case themselves, Mill had this devastating response: “Both teachers and learners go to sleep at their post as soon as there is no enemy in the field.”
If you follow this thought, you’ll see that it is not just the side that is excluded that suffers. In fact, the side that does the excluding suffers the most intellectually. A political monoculture sooner or later always degenerates into extremism and incoherence, because it needs an opposition to keep it healthy. Only your intellectual enemies have the motivation to pick off your weaker arguments and keep you intellectually sharp. . . . A lack of intellectual diversity hurts both left and right.