Ethnic diversity is increasing in most advanced countries, driven mostly by sharp increases in immigration. In the long run immigration and diversity are likely to have important cultural, economic, fiscal, and developmental benefits. In the short run, however, immigration and ethnic diversity tend to reduce social solidarity and social capital. New evidence from the US suggests that in ethnically diverse neighbourhoods residents of all races tend to ‘hunker down’. Trust (even of one's own race) is lower, altruism and community cooperation rarer, friends fewer. In the long run, however, successful immigrant societies have overcome such fragmentation by creating new, cross-cutting forms of social solidarity and more encompassing identities. Illustrations of becoming comfortable with diversity are drawn from the US military, religious institutions, and earlier waves of American immigration.

A team where everyone speaks a different language is very diverse, but not effective, since there is no communication. One in which they at least share some common language which for all of them is a second language, not with full fluency, would be able to function, but likely not as efficiently as one in which everyone speaks the same language as their mother tongue.

And language can be just an analogy. Instead of language, imagine a cultural background of shared assumptions and experiences. A shared background can help avoid misunderstandings and speed up communication.

At the same time, it's clear that diversity can help in many ways, avoiding single-mindedness, for example. If everyone has the same concepts and background, they might miss something important.

For political reasons, people like to say that diversity is good, and it is indeed good in some ways. But that's far from the whole story.

It would be better to not use "diversity is good for business" as a slogan, when there are other more valid reasons to fix issues with inclusion where they exist.