So what. Assimilation was no panacea(only a liberal would think that). Plenty of immigrants lived in unassimilated neighborhoods growing up and achieved success. Segregation does not equal failure. Thomas Sowell has written extensively on how much better African-Americans were doing scholastically prior to forced integration.
There is a big difference in choosing to live in a neighborhood with others with a similar background and being forced to stay within a restricted neighborhood, be forced to using separate bathrooms and water fountains lest you "contaminate" decent white people, and even be prohibited from voting. White immigrants may have lived in ethnic communities for the first generation or so, but society at large made it their business to assimilate them into mainstream culture, and by the third or fourth generation people by and large moved outside of those communities to better neighborhoods with a mix of ethnicities. All Italian, or all Irish communities are a rarity today outside the Northeast or some large cities. Most of those descendants of the original immigrants see themselves as just plain white people, or just plain Americans. This was called the Melting Pot, and it was for white ethnics but not for blacks.
Assimilation is important to the point where everyone living here feels invested in America and its ideals. Without assimilation, where some people are made to feel that they are not true Americans, or that they merely "guests" of this country, when the chips are down people will not come to each other's aid. Without a common culture, there is nothing it for you or I to protect this country, or to maintain its values or system of government.