Seriously though, you can use neo-Nazis to justify censorship, but then what about the less extreme alt-right? How about the slightly less extreme religious right? How about economically right-wing libertarians? Neo-conservatives? Where do you draw the line?
As I mentioned a few days ago on a related article, https://news.ycombinator.com/item?id=15122311 , the extremists are good "canaries" for indicating the gradual erosion of freedom of speech. When the edges collapse, the line moves just a slightly bit closer to the mainstream, and a new set of views are considered extremist. Repeat until everyone is brainwashed into a monoculture. That's the scariest part.