I think there are philosophical arguments for free speech that can be evaluated on their own merits:
1) If some idea is true, I'd like to be able to learn that it's true.
2) If a system silences a dissenter, then other dissenters (who could have better arguments) will know that there's no principled protection for them, and won't speak.
3) If some idea has been silenced, then most people don't know the best arguments for it, so they can't in good conscience support the silencing.
4) Clearly false ideas don't need silencing. Historically, an alarming proportion of silenced ideas have been true, but dangerous to the prevailing power of the day.
1) If some idea is true, I'd like to be able to learn that it's true.
2) If a system silences a dissenter, then other dissenters (who could have better arguments) will know that there's no principled protection for them, and won't speak.
3) If some idea has been silenced, then most people don't know the best arguments for it, so they can't in good conscience support the silencing.
4) Clearly false ideas don't need silencing. Historically, an alarming proportion of silenced ideas have been true, but dangerous to the prevailing power of the day.