Here is an impressive argument presented in Charles Blow's recent column: http://www.nytimes.com/2009/04/18/opinion/18blow.html

This matter came up in class this week as we were discussing Free Speech guarantees. I asked if they were willing to extend the right to groups that spout hate. Save for s student from Germany, the vocal participants think you can stamp out hate by suppressing voice. My German student explained how that strategy failed in her country in the 1920s, and is failing again today.