It's bad cause the people they bring will never get along. It creates a society where the inhabitants become slaves of lower wages, too, since usually foreign workers are hired because they work for lower wages. Everyone is also fucked over because all the cultures begin to "confirm" into a global consumerist culture (See America; Why do irrelevant European countries all of a sudden think of American Pop Culture to be something worth living under?)

Totality, it's bad. The result is identity politics. I can tell you about how this was all supposed to happen, but nah.