In sum, free speech is not intended to protect benign, uncontroversial, or inoffensive ideas. Those ideas do not need protection. It is intended to protect – to foster – exactly those political ideas that are most offensive, most provocative, most designed to inspire others to act in the name of its viewpoints. One could say that every significant political idea, on the right and the left, has that provocative potential. If speech can be constrained on the ground that it can inspire or provoke violence by others, then a wide range of political ideas, arguably the only ones that really matter, are easily subject to state suppression.
Other than would-be tyrants and their authoritarian followers, nobody should want that. Nobody should want the state to make and enforce lists of prohibited ideas. Even if such state action does not offend you in principle, there's a very pragmatic reason to oppose it: those who cheer when ideas they dislike are suppressed will very likely find, at some point in the future, that their ideas end up on the prohibited list, and will have little ground for objecting when it happens.
Emphasis mine. The author is not normally someone I quote, but his article bears reading in its entirely. Via Conservatives, Democrats and the convenience of denouncing free speech | Glenn Greenwald | Comment is free | guardian.co.uk.