Actually, I don't think the left has historically supported freedom of speech for fascists for precisely the reasons you (Charles) mention. That's why it has generally tried to confront and disrupt fascist rallies as a matter of self-defence. But at the same time it has objected to legislated curbs on freedom of expression because it has adjudged, accurately, that these are more likely to be used against itself in times of crisis.
^^^^ CB: Yes, in my papers and posts to this list on these issues over the years, I refer to your argument here as the "pragmatic" argument for giving First Amendment ( in the U.S.) protection to fascistic racist speech. There are pragmatic and idealistic arguments. The idealistic argument is refuted by the "absurdity" argument I just posted.
The pragmatic argument is essentially refuted or partially refuted by the facts of history, at least in the U.S. That history shows that Communists' supporting freedom of speech for fascists will not guarantee that the bourgeois state will protect Communists' freedom of speech. In other words, you are saying that we should support freedom of sppech for fascists, because by doing so , the bourgeois state will have to be consistent and protect Communist ( or Left) frreedom of speech some time in the future, because Communists have supported freedom of speech and organization for fascists.
But the KKK and other fascists had freedom of speech all through the period of 1919 through the 1950's. And all through that time the freedom of speech and assembly of Communists was not protected by the U.S. state . There were the first First Amendment cases before Holmes and Brandeis , et al, in which Holmes in his socalled famous civil liberties decision voted to leave in _jail_ , I think it was Gitlow, or the other one. In Holmes', "shouting fire ,falsely< in a crowded theater" opinion, he cites shouting fire , falsely, in a crowded theatre as an example of speech that is _not_ protected by the First Amendment. It is an _exception_ to protected speech; and uses he the analogy to put socialist-left anti-World War I protestors in prison ,or to leave them I prison under the lower court decisions. Their anti-war speeches were exceptions, like shouting "fire",falsely, in a crowded theatre, to protected speech. Then a few years later, Brandeis writes a paen to free speech ( as my Constitutional law professor put it) and then votes to leave Comrade I forgot her name for now, she was a Communist in Californis , leave her in prison for advocating Communism. In there about the same time were the Palmer Raids. Then later the Smith Act convictions.
At no time in this period were any fascists, KKK or even Nazis I believe, jailed in the U.S. for their speech. Henry Ford had a picture of Hitler up in his office.
So, I am saying that in fact respecting fascists' freedom of speech, not legislating against fascist speech, has not resulted in protection of Communist or Left freedom of speech "in return." The pragmatic strategy has a poor history. There was no "quid pro quo" : We oppose outlawing fascist speech, and the bourgeois state will not violate Communist or Left speech in "exchange".
Do not rely on your respecting fascist "freedom" of speech to stop the bourgeois state from repressing left speech if and when the bourgeois state decides it is necessary to repress left speech. The bourgeois state. won't let you go back and say "look, we didn't advocate outlawing fascist speech back then. You must give us the same in return. " They are treacherous.