Sure it's been said already but he is ignoring context, by saying a white person is unable to say the word he's basically saying white people shouldn't be allowed to make slave movies, unless he thinks there should be a slave movie without the n word? or to have it bleeped?
This n word controversy is essentially suggesting white people should 'stay in their lane' and blacks should stay in theirs without crossing into each others history/culture/identity, or at least not too far (by using the n word, but what is the limit? And who decides this limit?). It is regressive to start barring certain kinds of people based on the past and not the intent of people in the here and now. Birth of a nation should not be judged to be the same as Django because they were both made by a white guy.
As for Spike, I love him as a filmmaker and think he's a good person who means well but I think he's very jaded/hurt by the hollywood system, which he likely has a right to be. Black directors/lead actors are still marginalized but I think Spike is failing to realize it is more about money now, his movies don't make back their investment. The actor thing is a separate issue.
I get the impression he's upset that Quentin is liked by black people and black people go to see his movies more than they do his own, but Quentin isn't someone like Tyler Perry so Spike can't rag on the quality of his work as Quentin is actually talented. Spike does what he feels is necessary but there is a link between not being financially viable/critically acclaimed and the amount of bridges he burns, it is the shitty side of hollywood.
That being said I don't like Quentin's usage of the word in Pulp fiction, it makes me cringe to hear it. And I'm not into buying the excuse that 'bad people' are using the word, therefore it's okay because Quentin is showing how bad people are ignorant and so on. I'm inclined to give him this argument for Inglourious Basterds (nazi's were racist) or Reservoir Dogs (italian/irish mafioso racists), but not for Pulp Fiction.
It seems like for some reason Quentin just likes to use it without purpose. He isn't racist but it's still somewhat annoying, maybe he just feels like it makes his work more authentic if he's willing to go that far, like it's some protest against society saying he shouldn't use that word. I don't know.
I think I can sympathize with the view of Daniels and Lee. I think the root of the frustration may not be so much with the language Tarantino uses as it is the presumptuousness that he is the person to tell this kind of story. Even though Django is about the revenge of a slave, it's a pretty cheap film about slavery, substantively speaking.
This is one thing I'll give to Spike, I completely understand why he would be offended by a slavery movie being a 'fun' action adventure, and that being done by a white guy.