My friend and critiquing buddy Nick Mena has just interviewed a couple members of our critiquing group, Nyki Blatchley and Daniel Ausema, about diversity in their own work and in general.
Relatedly, Nyki has just included a post on his own blog which discusses one the most profound and important reason for including diversity in one's work: realism.
Coincidentally, author Malinda Lo said something very similar in her own blog today.
We live in a complex world that has a variety of people in it and always has. There's no reason to suppose that this wouldn't be true in a fantasy or SF world as well (unless, perhaps, you're writing a dystopia where everyone is cloned or something). The idea that only one *kind* of person (or culture) has experiences worth writing about is as ludicrous as it is offensive.