I don't think the Nazi's crimes were unprecedented, except in scale. The main difference from previous pogroms and ethnic cleansings was that the technological means for mass extermination were available for the first time- the transport, communications, and industrial processes.

I believe that had they had the means, the Spanish would have exterminated the Jews and Moslems in the early 16th century, rather than expelling or forcibly converting them. Cromwell would have possibly exterminated the Irish had he had the means. Even in the Bible, the Israelites were instructed to exterminate "the Hittite, and the Girgashite, and the Amorite, and the Canaanite, and the Perizzite, and the Hivite, and the Jebusite", and that they failed to do so is no exculpation.

The ethnic cleansing in the former Yugoslavia, mass murder in Rwanda, and recent racist attacks in Kenya show that whatever they call themselves, people can still succumb to the attraction of Nazi style short cuts to social problems. We must always be on the lookout for this. It's possible that use of "Nazi" in other contexts devalues the word, but equally possible that it refreshes the memory.