Max Hastings makes some interesting points about the recent controversy when Nick Griffin and David Irving were invited to speak in Oxford:
Muslim extremists say worse and more dangerous things about Jews than Irving ever has...Not just about Jews, by the way. Radical Islam is a superset of Nazism and Fascism.
There's another angle on this, though. If some speech is to be prohibited, who makes the decision? Who would you believe has the perfect knowledge of the past, present and future - and of the universe and everything in it - necessary to decide whether an assertion is, or will be found to be, true?