The argument for ideological diversity on campuses is strengthened by the growing political polarization in society. Political polarization is costly, because citizens then are more likely to dismiss a policy position based on the identity of its supporters and opponents than on the merits. Polarization also makes it harder to reach compromises, and compromises are more often likely to lead to political stability than ideas with a more narrow range of ideological support.
One of the reasons for polarization appears to be that citizens today are more able to live in ideological and partisan cocoons than in the past. They can look at the websites they like and not at those that might challenge their views. Cities and towns also sort themselves out more by political beliefs. Those opposing the predominant views of the their current residence are more likely to move to a more politically hospitable climate. Apparently, Republicans and Democrats even choose to follow different celebrities although they do admire a golfer or two in common.
The most obvious place where citizens should learn to interact with ideological opponents and confront arguments that will challenge their views is the university. But this experience is less likely if the gatekeepers of ideas are almost uniformly of one political persuasion. And so many of our modern universities are ideologically monochromatic.