There is an argument in favor of free speech based on the importance of being free to make a decision, having all the related information available to you.
Consider a society in which we decide, by vote or using the market correspondingly, to whom political or financial power should be assigned. Shouldn't we know what these competitive individuals like or dislike, in order to choose? If, for example, I dislike X group of people, being not allowed to say so means others will never know I hate X. Shouldn't X and those supporting X's ideas know my feelings?
In a free market of ideas expression of hate speech leads in individuals, aware of one's ideas, able to decide rationally who to empower with their vote or money. If expression of sexist, racist or homophobic feelings was forbidden, we would never actually know enough, in order to make a political choice. A sexist might thus be voted to deal with woman's issues. If “Barilla” was not allowed to express itself against new forms of family, homosexuals' or one-parent families would be deprived of the option not to put “Barilla” on their table.