So, I've been thinking about the whole diversity thing a bit of late...
I don't think we really have a very good understanding of what "intelligence" or even "reason" truly are... We have woefully inadequate knowledge about how the brain works, even less how the "mind" works.
So here's my theory, which is probably controversial...
Much of what we determine to be "intelligence" is based on highly evolved pattern-matching algorithms. The ability to quickly relate new experiences into known patterns, the ability to pick and choose from multiple patterns to integrate a response to situations that may not directly echo a pattern you previously know, etc. Those who we consider most "intelligent" are just simply better at this, their brains do this more effectively and briskly, than those who are less "intelligent". Some of the most "intelligent" are those who are able to see something that triggers pattern-matching but are able to recognize when something excludes that pattern's relevance.
The purpose of education is to help us by exposing us to a very wide array of possible patterns for future use. General education gives a baseline. Specialized education [such as science/engineering, medicine, law] are no different, in that they deal with topics so specialized that you need training to even get a baseline of patterns to even get your foot in the door.
That said, we integrate patterns throughout our entire life, not just via formal education. Many patterns are dependent on our upbringing and social/cultural forces of our place in life. Thus, the patterns that I recognize, as a white, male, middle-class, educated white-collar worker from an upper-middle-class very WASP-y Chicago suburb, will be very different than someone who perhaps *looks* like me and has the same nationality makeup but grew up on the south side of Chicago, only completed a high school education, particularly a parochial school, and went on to trade school to be a plumber. And it will be light-years different from, perhaps, the child of Cuban immigrants in South Florida who grew up in a primarily Spanish-speaking household and fought through that diversity to go to college and law school and is now an Appellate Court Justice with a potential nomination to SCOTUS.
Here's where pattern matching can go wrong... If you have certain patterns in your brain, it can sometimes be nearly impossible to see things outside of the patterns you've learned. I'll tell you from experience, that some of the smartest engineers in the world can turn into some of the worst when they're trying to solve a problem that resembles a problem they've solved before but has a completely different root cause. They will beat their heads against the wall believing it's the previous problem, and when you finally find the real root cause, you have to beat them over the head with it to get them to see that it wasn't what they initially saw. It's REALLY hard to treat every problem as a new problem, and to try not to anchor yourself to past problems, because in a world where we all have 100 things going on at any time, it's easiest to simply classify it as what you know, first.
The same thing can happen on SCOTUS. If all of SCOTUS are white male upper-class Harvard/Princeton grads, they are going to have a hell of a lot of smarts but likely have a relatively narrow group of patterns to match. Something that exists outside that pattern may not be impossible for them to handle, but it will likely be impossible for them to SEE without getting clued in by someone else whose pattern matches.
Diversity of patterns is not encompassed by race, or sex, or whatever "external" category we can identify by sight... But those are often a proxy for very different patterns. And bringing different patterns to any problem-solving endeavor (of which SCOTUS is entirely one) will be important given the extraordinary diversity of cases we ask them to decide.
Which is to say that there is value in diversity. Such that if you have two equally-qualified candidates for a slot on SCOTUS but one has the same upbringing and looks exactly like the other 8, and one has significant diversity of pattern through life experience, race, etc, diversity would be a virtue for diversity's sake.
