"Every man who says frankly and fully what he thinks is so far doing a public service. We should be grateful to him for attacking most unsparingly our most cherished opinions."
-- Sir Leslie Stephen
(1832-1904), literary essayist, author
Source: The Suppression of Poisonous Opinions, 1883
Surely the above is at least a foundation of democracy - perhaps the primary one. Ideas and opinions are not worthy of respect. As soon as they're uttered, they deserve nothing more than to be attacked, beaten, defiled - even as we maintain our respect for the person who expressed them. The worthy ideas will survive and flourish. Not all ideas are created alike, and the fact that someone holds an opinion (or belief, or creed) has no bearing on its value.
That said, the same argument holds for the criticisms - they must be subject to purifying abuse as much as that which they assault. And that's where our democracy is most in danger of falling down - not in the plurality of opinion, but in weak, impoverished critical analysis of those opinions.