Put succinctly: debates are the reason for our freedom to speak (and debate)
The cornerstone of the democratic process is that we have an open marketplace of ideas. That a well informed populous which is able to discuss, debate, and consider matters of public importance - even if some of those viewpoints are unpopular (or even seem absurd at the time). From those discussions we choose our leaders, as well as interact and advise them (write your congressman!) from time to time.
Some quotes:
"We can never be sure that the opinion we are endeavoring to stifle is a false opinion; and if we were sure, stifling it would be an evil still. ~John Stuart Mill, On Liberty, 1859"
This is an argument against limiting speech - without properly considering an opinion, we cannot judge it right or wrong. Plus, it can be considered a human right to think as one will - and to have one's words reflect that.
"I am thankful for all the complaining I hear about our government because it means we have freedom of speech. " ~Nancie J. Carmody
Freedom of speech is the hallmark of a free society. One cannot exist without the other.
"We are not afraid to entrust the American people with unpleasant facts, foreign ideas, alien philosophies, and competitive values. For a nation that is afraid to let its people judge the truth and falsehood in an open market is a nation that is afraid of its people". ~John F. Kennedy
Again, the idea of a marketplace of ideas.
"I disapprove of what you say, but I will defend to the death your right to say it". ~Voltaire
Once again, freedom of belief (and expressing those beliefs) is seen by many as a human right.
Good luck.