I... don't buy it. I mean, I think Hunger Games Tweets (which Miller both mentions and links to) blows the Whitewashing Promotes Empathy theory out of the water. (I mean, the idiots who took issue with the casting of Amandla Stenberg did empathize with Rue... until they realized that her skin was brown. Which is the opposite of what the findings of that study suggest.)
I'd much rather see cover art (and movies, etc.) reflect the characters as written, rather than reflect what some entitled, semi-fill-in-the-blank-ist reader expects to see.
But I'm overtired and falling asleep, and therefore seriously lacking in the ability to be articulate or coherent.