By not allowing dissenting opinion, society remains structured and peaceful. The theory is that if we don't disagree, we don't fight. In order to open up society to all ideas, it costs a certain amount of order, and a certain amount of conflict. Following this line of reasoning, there is also the high cost of defending freedom above one's own ideas. One can firmly believe that pornography, for example, is harmful.
Freedom comes with a bill.
And just how much of it we get depends on how much of that bill we are prepared to pay. I've spent a lot of time in this column discussing censorship as an evil. Nothing, however, is always evil. Freedom issues in its own evils, and the price of having it is dealing with those evils. Still in all, I find that it is worth the price.