Freedom of speech is a fundamental, basic right in a democracy. It includes the right to be stupid, to say idiotic things, to lie, to wish ill on other people. It is not absolute, however. When it clashes with other basic rights it sometimes must give way.
For a while now there has been a major argument in the US about the invitation extended by Columbia University to the Iranian President Mahmoud Ahmedinejad to speak before the faculty and students of the university. Deborah Lipstadt, for instance, who knows a thing or two about Holocaust denial, has been against the invitation from the start. Denying the Holocaust falls beyond the limits of freedom of speech, and calling for the destruction of Israel from the position of the head of a government, even more so. Juan Cole, writing at Slate, predictably, frames the discussion in other terms. It's not about what you think, because "There is, in fact, remarkably little substance to the debates now raging in the United States about Ahmadinejad." (and anyway, "he allows Iran's 20,000 Jews to have representation in parliament". Wow!). Rather, Cole informs us, the real issue is the the Neocons and their ilk want to go to war with Iran, so they can't allow Iran's president to be heard. Katrina Vanden Heuval has a similar argument at The Nation. The scary (despicable?) things you're hearing from Iran aren't really serious; the real problem is what Cheney and ilk are saying - now that's evil for you (my paraphrasing).
Personally, I haven't had a clear opinion. It seems to me a pragmatic question: is the danger of silencing greater or lesser than the danger of hearing him? Winston Churchill knew how awful the Nazis really were from the very beginning, even when his was a lone voice in the wilderness, because he was listening to them - as were the appeasers all around him. He was listening and believing, they were listening and brushing aside.
Now that Ahmadinejad has been, talked, and moved on, I think, overall, that it was alright. The Coles of this world weren't listening anyway, because, as they say quite openly, they're too busy focusing on the Cheneyites. For Helene Cooper for the New York Times, on the other hand, the most compelling images were that Ahmadinejad insisted there are no homosexuals in Iran and that the Holocaust isn't fact, merely a debating point. If that's ultimately what remains in the public mind, then giving him the benefit of the doubt and allowing him the freedom to spout his rubbish was a good idea - as the framers of democracy thought it would be.