Following recent events in Milwaukee, NYMag summarizes some of the sociological research on urban segregation. As noted, the results of persistent segregation are not good.
However, what if this segregation is desired by the city or by large percentages of residents, whites in particular? The outcomes of segregation may be bad but the alternative might be considered worse: whites would have to live near blacks and share more resources and social spaces. With segregation, the problems are largely confined to certain areas (though there can be widespread fear if bad things happen in whiter and safer areas) and can be explained away by being attributed to particular groups. The segregation can be a self-fulfilling prophecy: there are neighborhoods for different groups because these sorts of things keep happening.
At some point, the negative consequences of segregation may be too much to ignore and stronger action might be taken. One good example is the destruction of public housing high-rises in many cities in order to deconcentrate poverty. City and national leaders perceived that something needed to be done. Yet, less provision was made for helping residents move elsewhere and find better opportunities. A less helpful example might be the uptick in shootings in Chicago: this is widely decried by leaders and the public but will much action be taken to (1) counter segregation and (2) provide more economic opportunities? At the moment, it appears not.
tl;dr: the outcomes of segregation are bad but how much will is there to consider alternatives?