That doesn't mean you have forbid Nazis from ever speaking (as if this would work), it means you have to make sure your democracy and your institutions are stable enough to survive their first period in government and manage to get a peaceful and fair transfer of power after that (instead of Nazis setting fire to the Reichstag, blaming it on communists, declaring an emergency and never have elections again).
I'd rather loose the right to utter certain inflamatory phrases than loose the right and the possibility to vote the next government out of office in a fair and representative election.