While this is true, the book will, over time, feel more and more anachronistic. This is inevitable and natural. I still think it's a good book to teach in 2017, whatever, but I can also see why the parents of a black kid may not want their children studying a book where the n-word features dozens of times, even if the book isn't exactly endorsing the slur. Also, while Harper Lee may have been progressive "for her time," the book is not without its problems or its own dated attitude.
You could argue that kids of color "need" to be exposed to it, or that they're going to hear much worse in the real world, or that the merits of the book outweigh any discomfort they may have reading the n-word over and over, even in a literary context ... but I can also see the other side of it.
My general feeling is that difficult or controversial material can be taught thoughtfully and intelligently and compassionately--and that good art tends to be discomfiting by its nature--but I also think it's important to listen to people when they say something upsets them. And I don't think we need to force a 60 year-old paternalistic white person's novel onto anyone as some sort of defining account of racial injustice. They could just turn on the news.