I agree, while a single campaign might take place in some small backwater, and thus feature a single ethnic group the world at large should be diverse. Not every PC in the books needs to look like a 15th century yokel from Burgundy.
Edit: Also, don't limit physical appearance to generic real world groups. Someone with dark skin, red hair, blue eyes, and Malaysian bone structure could be an interesting character.
The real world has far more than 3 ethnic groups. A fantasy world can have all of them and more.
There are some problems however. How do you mix race in the ethnic group sense with race in the D&D demi-human sense? Are there multiple ethnic groups for all the other races? You have problems either way. If all the elves are white, and dwarves are black, that's bad. If there have to be full representations of every ethnic group for every non-human race it gets rapidly ridiculous from a would building standpoint. "Okay the Bantu Orcs are the the hills north of Greyhawk. Where are we putting the Inuit Gnomes?"
(Side note: Technically speaking if a group can interbreed, they are a single species. IE; Men, Orcs and Elves are all the same species.)
It was a discussion of the Grumpy RPG column on Blacks in Gaming, that had in part drifted into art. It also drifted into real-world politics, and was closed.