Up until recently it was regarded as 'politically incorrect' to question multi-culturalism
Big difference between ethnically diverse and multi-cultural...
If people have the same or similar values I would say that any extent of ethnic diversity can work... but where people do not share or have polarizing views on issues intrinsic to an open and shared society such as social wellbeing, human development, methods of progress, egalitarianism, access to education, tolerance of hatred and violence, abuses against minorities, women, animals, abuses against freedoms, fundamental ways to achieve health, happiness and livelihoods... then minority groups may be formed through immigration... Understandable really.
Don't get me wrong, any amount of ethnicity can bring good things to a country, not every modern society knows anymore how to care for its sick and elderly and can be a little dismissive of other family commitments, as well as to the care and time given to other aspects of life such as faith and healing...
But if the presence of foreign inhabitants causes great consternation or actually *generates* violence for those involved, then I would say there *is* a problem and something has to be done to bridge the gap... or people will just learn themselves how to deal with it. :)
But there are always good things learned from polarization too... we didn't come this far as a reasonably tolerant and open minded species without being able to adapt to *some* change... And where these changes present challenges, reflects the society into which those changes are being introduced more than anything else... ;)
In any case, they are mostly just teething problems and will continue to challenge and change societies worldwide whether we like it or not.
Basically what i'm saying is that people don't immigrate between ethnicities.. they immigrate between societies, and those societies are formed from shared beliefs and commonly held values... :)