So, if we have a group of people that, are broadly speaking, all the same type and we think adding someone with a different set of characteristics will lead to the group performing better as a whole...we shouldn't do that? We should instead higher the "best" individual regardless of how it impacts the group?

The goal is to get the most productivity from the team/group. Sometimes that means hiring for attitude over aptitude or ...oh yes, diversity - be it race, gender, degree, life experiences, etc.