For some categories, it's probably harmless. I don't think we need to re-sort living arrangements until we have a proper diversity of sports fandom, for instance.
However, the most prominent understanding of "difference" in the western world is race. Racial hierarchies have led to unequal distribution of resources, unfairly balanced prospects for individuals and unequal representation in government. Arguably, race has been weaponized to disenfranchise a large number of human beings, only some of which are members of the "lesser" race. For instance, scholars have argued that racial hierarchies allowed landowners in the post-civil-war-south to avoid material compensation of their white workers because the workers were satisfied by the 'comparative gain' of being better off than working class african americans (who the same landowners paid even less).
Although your precise wording is correct ("prominent understanding"), far greater splits exist, for example, on political views and on income, and arguably even more harmful.
I focus on race because it's an entirely artificial construction. It makes sense that, say, communists and capitalists might want to live in different communities (and it's not clear it's a bad thing). There are also forces that combine to push people of different incomes into different areas that aren't related to a desire for cultural homogeneity.
At least where I live, the segregation isn't on skin color, although it sure looks that way at first glance.
There are two major communities (African Americans and whites) and a number of smaller ones.
The "white" community actually refers to "WASPs and their descendants," but a number of other people get lumped in based on skin color. The AA community refers to descendants of slaves.
The two communities speak different dialects of English. They have drastically different communication patterns (when do you use last name versus first name? when is it okay to yell? how do you show respect?). They have different religious practices. They have different practices for raising kids. They work different jobs. They differ, significantly, on almost all cultural dimensions.
I see very little discrimination or desire to segregate based on skin color. If an Ghanian moves into the community, they're more likely to move into the white community than the African American one. Likewise with African Americans who grew up in WASP communities.
In practice, many immigrants -- of all skin colors including whites coming from places other than Western Europe -- tend to form their own community; they don't quite fit in in either the "white" and the "AA" community. Both communities accept a narrow range of behaviors as appropriate; the broader international community tends to give a bit more cultural leeway. A Nigerian, a Japanese, a Russian, and an Indian immigrant will interact with each other just fine, but quite often don't assimilate too well into either "white" or "AA" communities.
Distinct cultural communities that often, but don't always, correspond to ethnicity is exactly what you would expect from the construction of a "racial" hierarchy.
If we divided caucasian Americans into two groups, designated one group as "lesser," and then waited a few hundred years you would find similarly stark cultural differences. Again, race is entirely artificial. People were divided based only on superficial appearance and, once those divisions are enforced, cultural distance grows.
So, what you're experiencing is the outgrowth of a racial classification system. It's entirely artificial but, as you say, it also has real consequences. Even if, by magic, we were all transformed to have the same ethnic heritage, the cultural legacy of "race" would make "race" very apparent. African American and caucasian American culture is different - even though those differences are partially (largely? hard to say) from the racial structure imposed on African Americans.
The answer is in your final paragraph. Neither "white" or "AA" culture is a perfect fit for new arrivals (just ask Jamaicans or darker skinned Dominican ex-pats how well they feel seen by any American culture). Indeed, no single cultural approach is "right" and we do ourselves a disservice by searching for one. We have had structures and policies that have driven the two largest cultural groups (white and African American) apart for generations. Abating that push should be a high priority. We also need to be attentive to including as many cultural groups as possible in finding common ground and understanding for living together comfortably. All groups will need to "give" a little ground in terms of what they would most prefer and, in the interest of equity and facing history, many experts think caucasian Americans should give the most.
Because segregation permits large organizations, like the state, to easily provide differing classes of services (or sets of rights) to the different, segregated groups, again with small biases having outsized systemic consequences.
Example: when I was growing up, the original 313 Detroit area code broke off the then-new 810 (and later 248 and 586) zone, leaving only the city of Detroit and the west side as 313. To a majority of the metro area, after that split, non-810 denoted non-white. It allowed people who might not otherwise be able to discriminate an easy tool to do so, and some businesses would implement bias against customer database records that had area codes that strongly correlated with race and socioeconomic status, furthering the historic divide. Prior to the area code split, this wasn’t as easily available to those who might do that.
It furthers systemic inequality, and some of the core tenets of our society are that we are all equal before the law and all deserve a level playing field in the market.
Wouldn’t it be easier in some ways to suppress a minority if they are just scattered about through the general populace and have a tougher time developing a community and addressing common concerns and interests?
> What pattern of exploration and social learning produced the best outcome? We discovered the answer when we plotted the return on investment each trader got against the diversity of ideas he or she harvested through social learning.
Social physics supports a framing that groups are like "machines for finding the best ideas", and having group members with access to diverse sources of ideas (polygons from different spaces and experiences count as this) is part of a properly calibrated social network where the best ideas are within reach, to address any given challenge that may surface.
