Free speech is a prisoner's dilemma. The prisoner's dilemma ("PD") is a game theory paradigm involving two players and a series of payoffs for betrayal and cooperation. The highest payoff in a single round game is obtained by Player A if he betrays B while B cooperates with him. B receives the lowest score, known as the "sucker's payoff". However, in a multiple round game, B will betray A at the next opportunity. A soon figures out that the best score he can obtain in a multi-round game results from cooperation in each round.
Think of an act of speech--an utterance--as a move in the PD. In the default state of human barbarism (and through-out most of "civilized" history) any given utterance was regarded by somebody as a betrayal. Since "tit for tat" is the most successful strategy in the PD, the likely response was also an act of betrayal. Of course, in any series of betrayals the actions involved will escalate; this is true of nuclear confrontations, conventional wars, and human activity of all kinds. To bring the game to closure, I will hit you twice as hard as you hit me, hoping to be the last one standing.
In a series of encounters beginning with an unwelcome utterance, one player will inevitably escalate from speech to action. You hurt me with words, I hurt you with rocks, flame, or bullets. Although we don't routinely think of these as speech-related incidents, most of the really famous killings in history, including the execution of Socrates, the crucifixion of Jesus, and the assassinations of Mohandas Gandhi and Dr. Martin Luther King, were responses to speech.
In the free speech version of the PD, the first step is for the players to agree to limit their betrayals to speech acts. This is fundamentally similar to other human accomodations involving the renunciation of chemical weapons, nuclear arms limitations and the like. We reach an agreement that no matter what one of us says, the other will respond only with speech, not with bullets.
A very interesting state of mind results from this. If we keep it up long enough and are successful in renouncing force, at some point we will stop regarding each other's utterances as betrayals, and we will regard them as acts of cooperation instead. We will have created a system in which your ideas and mine, no matter how diametrically opposed to each other, are freely offered to one another as moves in a system we both welcome. From John Milton and John Stuart Mill on, writers and philosophers have listed the rationales for allowing "bad" speech: it may turn out to be controversial "good" speech and even if not, it highlights the good speech, etc. The PD offers a more fundamental basis for understanding freedom of speech. We mutually renounce the supremacy of our own judgments, enforced by acts of physical retaliation. Across a series of encounters, we each receive the benefits of cooperation. The benefit is the utterance of our own ideas without fear of violence; the price we pay is having to listen to the other player's ideas without engaging in violence.