I've made this point before, but I feel compelled to repeat it in response to this post. The late Christopher Hitchens, in one of his greatest orations[1], made some excellent points about free speech:

...it’s not just the right of the person who speaks to be heard. It is the right of everyone in the audience to listen, and to hear. And every time you silence someone you make yourself a prisoner of your own action, because you deny yourself the right to hear something. In other words: Your own right to hear and be exposed is as much involved in all these cases as is the right of the other to voice his or her view.

In other words, freedom of speech is really about freedom of hearing. Later, he asks who should be trusted with the job of censoring:

Who's going to decide? To whom do you award the right to decide which speech is harmful or who is the harmful speaker, or to determine in advance what are the harmful consequences going to be, that we know enough about in advance to prevent? To whom would you give this job? To whom are you going to award the task of being the censor?

Did you hear any speaker in opposition to this motion (eloquent as... one of them was) to whom you would delegate the task of deciding for you, what you could read? To whom you'd give the job of deciding for you, relieve you of the responsibility of hearing what you might have to hear? Do you know anyone- hands up- to whom you would give this job? Does anyone have a nominee? You mean there's no one in Canada good enough to decide what I can read? Or hear? I had no idea... but there’s a law that says there must be such a person. Or there's a subsection of some piddling law that says it. Well to hell with that law then. It's inviting you to be liars and hypocrites and to deny what you evidently already know already.

And finally, he notes that religious texts contain passages that would need to be censored, yet religious conservatives often call for censorship.

Look anywhere you like for the warrant for slavery, for the subjugation of women as chattel, for the burning and flogging of homosexuals, for ethnic cleansing, for antisemitism, for all this you look no further than a famous book that's on every pulpit in this city and in every synagogue and in every mosque. And then just see whether you can square the fact that the force that is the main source of hatred is also the main caller for censorship. And when you've realized this you'll therefore this evening be faced with a gigantic false antithesis. I hope that still won't stop you from giving the motion before you the resounding endorsement that it deserves. Thanks awfully. Night-night. Stay cool!

(If you haven't watched the whole speech, I recommend doing so. I've only transcribed a few choice passages, but the whole thing is great.)

And of course, there's the absurdity factor. Nobody thinks that censoring actually stops people from endorsing verboten opinions or talking about banned topics. Everything just gets driven underground. This gives the holders of censored beliefs an excuse to behave more extremely than they otherwise would. After all, they're being persecuted.

1. https://www.youtube.com/watch?v=QIyBZNGH0TY#t=1240