Freedom of speech is the center pillar of any civilized society. It guarantees that one is free to think without any boundaries and is free to express those thoughts in anyway he/she likes to.
If not for freedom of speech ideas would never get the proper checking and criticism, this transforms society into fertile environment for thought and progress offering the full potential of the entire society's collective intellectual power.
Sure the corruption and brutality of dictatorship is among the reasons we revolt but my main driving force towards revolution is the oppression of my words.