While I'm not in favor of banning racist speech, I should say that unlike some here I'm not blind to the consequences. I appreciate that you have the best of intentions and you may even believe what you're saying, but not every moral position can be backed up by a practical argument - in the end, we have to decide what kind of society we want to live in, and whether we wish to live our values even if, on occasion, there are negative consequences.
Racism can and does have deadly consequences, and the free, unchecked, expression of racism can, does, and will in the future, allow those who'd otherwise avoid going down certain roads knowing social ostrification follows, to follow a path that leads to discrimination, violence and death, directly or indirectly. The rise of Hitler, or conversely the enforcement of the constitution against the south, did not have zero effect on the amount of non-state-sanctioned racial violence.
As a basic example, over the last 12 years I've seen an alarming increase in the amount of anti-Muslim hate speech. This has translated into acts of violence and even terrorism against ordinary, non-violent, Muslims (or people idiots think are Muslims like Sikhs...) It's hard to believe that without a body of people claiming that most Muslims are anti-American terrorists, shored up with a litany of often dubious, and frequently irrelevant, attacks on Islam, that this degree of violence would be occurring.
Should those who promote Anti-Muslim hatred be jailed? Of course not. That would be to undermine our values and what we stand for. But our values are successfully abused by evil people, and we shouldn't pretend otherwise, and invent bogus "practical" arguments to defend our values, which have always been moral, and moral alone.