Many years ago when I was a teenager, I saw a version of Hamlet with a black actor in one of the leading roles. Knowing that the film was set in Medieval Denmark, I had a moment of being jarred out of the suspension of disbelief necessary for effective drama. However, I made the mental effort to thrust the issue aside and found no problem enjoying the play. Such casting has apparently now become common, and the reaction of viewers is now standard: we have decided not to care.
In fact, it turns out that Medieval Denmark -- if it were much like Medieval England -- probably did have a certain number of people of color in it. Several of the Round Table knights turn out to have been, a fact missed because the Medieval authors didn't make a big deal about it. It apparently wasn't that remarkable.
Nevertheless, one can see that the suspension of disbelief is a real issue for dramatic works. More than that, if a play is really partially about race, the dramatic vision of the whole could make it valid to cast certain roles in a certain way. It might be interesting to do a version of Roots or Tarantino's Django with racial casting reversed, but it makes a certain sense to play the casting straight. I'm not sure it would make sense to fail to take race into account at all: while reversing casting would make a point, you would lose something important to the drama if you gave the sense that race was of no importance in the time period being portrayed. It is the centrality of racism that is the issue, and casting has to reflect that somehow.
I'm thinking about this because the Hamilton musical has come under fire for a casting call asking for only "non-white" actors. There's a question about whether the law can support race-based casting. That's a separate question from whether or not it makes any sense to try to put on a play set in a particular time and place that doesn't take audience expectations into account -- either to smooth suspension of disbelief, or to challenge their preconceptions.
It seems as if the artistic concerns are valid, but they may be illegal. If they were illegal, should there be a nondiscrimination exception for art?