Whilst I agree in general with Pinker's defence, I think his description of free speech as "fundamental" in its importance is not quite right, but that it is close enough for the purpose of emphasizing the political importance of freedom of speech.
Of yet more basic importance is conceptual thought and its' expression and formation through language. Political correctness imposes at least two restrictions on conceptual thought. The first is the direct restrictions via social or legal censure, and it is these restrictions to which Pinker is largely addressing his defence of freedom of speech. However, the second type of restriction is the corruption of language, of which the very term "hate speech" provides an illustrative example. Simply putting the two words "hate" and "speech" together absent context would seem to integrate the two concepts so as to mean all speech that contains expressions of hatred when in fact it is only intended as a tool for control of social and political discourse via stigmatization and punishment. A white, heterosexual male who says something rude to a black, homosexual female may be guilty of "hate speech", but never the other way around. The tool of "hate speech" has not only a social and legal aspect, but also an anti-conceptual, anti-epistemological aspect. This becomes apparent when we think not of those using the term "hate speech" as a political tool of discourse, but of those people (perhaps young students in their formative years) trying to use the term "hate speech" as an actual concept with which to think and understand the world around them; since the term is not intended to integrate all expressions of hatred, it is wide open to contradiction and thus likely to lead to confusion, misunderstanding and rancour. But then, that's almost certainly what the originators of the term intended.