At Gaylaxicon, an author (I believe it was naomikritzer) told a story about how she just decided to make a female protagonist in one of her stories Hispanic because...why not? She did a little research into names so she wouldn't look stupid, and voila. Later (I think she had read the story to a class of kids), a young Hispanic girl approached her and told her how much it meant to her to hear a story where the heroine was *just like her*.
Did the Hispanic-ness of the heroine inform the story? Sounds like probably not. Did it make a huge difference in the life of at least one reader? Sounds like yeah. I know which side I'd rather err on.