Diversity is good to have in society. It helps people learn to get along and cooperate better, especially in the workforce. It teaches people that we are all different and helps us learn about other cultures. Diversity brings a variety of different opinions and viewpoints to the table and does a lot of good socially.
Diversity in society is what creates community. Growth and other aspects that promote well being all happen through diversity. Exploring cultures unknown, experiencing new identities, and being open to both of these things are indeed valuable to have- and should always be present. If diversity was not allowed and wasn't considered a valuable social good, prejudice and other forms of control would be harmful.
The fact is a society is formed by like minded individuals. Throughout history, homogenous societies have had a higher civic engagement, more rights, more prosperity, less poverty, and less crime. Compare the problems we see in diverse South Africa, some cities in the US, and multicultural places in general and you will see that diversity is not a strength, but a problem to be overcome.