Diversity is important.
Now, I don't believe in tokenism, but I do believe in representation. As a straight white male, it is all too easy for me to be blind to the way my 'type' is over-represented in fiction. That most fiction seems to be directly catered to me, to appeal to me, to feature me.
But having diversity among your characters is not only about representation; it's also just good drama. What your characters look like might not seem superficially important (in text there is no color) but even if all of the characters are white (or black, or Hispanic, or desi) it is important to make each distinct.
Basically, the less variation in the characters' cultural and developmental backgrounds, the less inherent drama there is in their interactions. Why limit yourself? No matter how well you define your individual characters, if they all share so many of the same characteristics then there is much less scope for clashes.
For me, as a function of where I live, it is easy for me to create a naturally diverse cast. I simply look at my friends and co-workers and draw from them - as they are from various backgrounds and ethnicities, so will my own characters be.
I especially think about the teenagers I work with, since the Sleepwar Saga leads are all teens as well. As I look to real young people I know from which I can extrapolate brand new fictional versions, I naturally will create a diverse group as that is the culture that surrounds me. Were I still living in some of the more overwhelmingly white areas I have lived in before, perhaps I would have to work that much harder to ensure I created a realistically and entertainingly diverse cast of characters.
That's not to say I always succeed in my efforts. Only after the fact did I realize that all six of my leads (and the supporting cast of "Straw Soldiers" as well) are cisgendered hetereosexual able-bodied people. Clearly this is an area I will need to deliberately focus on if I want to be representative (and to mine all resources for inherent drama).
Actually, one of the characters in "Straw Soldiers" is gay, but as it never came up in the text no one will ever know.
What about gender? More than half of the world's population is female, so why are so few fictional characters female? Especially in movies and on TV, but books can be just as guilty.
I want to be diverse. I want to be representative. Not just to ensure that straight white men are not the only (or even primary) type that gets stories written for them, but because when drama is all about the way characters clash and interact, why would any writer remove potential for this by limiting the variation among the core cast?
But while sometimes this comes easily, at other times I realize how far I still have to go if I want to meet my goals. It's so easy to surround yourself with people like yourself - to write about people like yourself - that you just don't notice how insufficient this representation actually is.