Without open debate, the public won't be able to discern good ideas from bad. It is only in a debate that you can understand the reasoning behind an idea.
Discussion
Free speech would allow for dissemination of good ideas as long as they are understood and persuasive. In days of old people stood on crates in the public square doing just that and would create huge movements.
Debate suffers from the irrational urge to support our team: right or wrong. Today there are mostly just political debates, and people rarely are mind changed from these events. One could make the argument that debate is designed to do just that; stir up the base.
Either way, it's not enough to be right. One has to be understood and persuasive.