The first point, while valid in many circumstances, ignores the fact that the word can be used, in the right context, in a non-offensive manner. For example, we could be talking about pre-Civil Rights Movement America, and how white men didn't want their daughters dating a "dirty nigger." Does that word, specifically, need to be used? No, but it can be use to highlight the deep-seeded racism of certain eras, people, and ideas. It may not be the best idea, but it works, and to be offended by it, in that context, typically means that you really haven't been paying attention at all to what the other person has been saying.