I love the game. I think the most valid argument against including modern minorities is staying faithful to the author's vision and what is in his books which are being used as source material for these games.
That said as someone who has done post-graduate work in European history I've got a big problem with the history argument. I hate hearing this argument being made for defending the games. It's weak. It's grounded in only the most shallow and superficial understanding of European history or the culture and society of Medieval Europe.
Medieval Europe was not as closed off from the wider world as some would like to believe. The African Moors ruled most of Spain until late in the middle ages. In the Slavic world they would have certainly been familiar with Tatars, Mongols, and they were certainly using the river system in Russia to navigate to the Mediterranean and trade with the Arabs or the Persians. This idea that people with dark faces would have been unknown to white Europeans of the middle ages is preposterous. Certainly the larger towns and ports would be more cosmopolitan and the educated classes and the elites would have been aware. It's a weak argument so stop making it.
Lastly, despite the medieval flourishes and touches this world is entirely fantasy so the idea of black people or asiatics walking through it is not as preposterous when set against having to fight griffins and harpies, is it?
This whole idea that fantasy games with armor, swords, and shields etc should be populated with only white people is ridiculous and absurd. You have to realize how silly you sound to any rational person when you say "I want a medievalesque fantasy game populated by elves and dragons but I will have a gran mal hissy fit if they put a black person in it!!" Enough with that!
The only valid reason for not including black people in this game is if it's faithful to the books which I confess I have not yet read. But I'd accept that argument before I accepted some preposterous, pseudo-historical argument that is entirely ungrounded in historical realities and absurd to tie to fantasy game to begin with. My two cents and I realize I am probably wasting my time with this. It's a great game. Relax and have fun with it. Even a good many of the people criticizing it recognize it's mastery. You don't have to close the gates, blow the horns, and pour hot oil on any negative comment.

It's not worth it.