The NYT has a really thoughtful piece by Frank Bruni on the self-defeating overuse of hyperbole in political discourse. I recommend reading it in full, but if you’re in a hurry, I’ll try to sum it up for you: The use of absurdly overblown comparisons makes constructive conversations about issues impossible and, by turning every analogy up to eleven, the noise level is so high that it’s difficult to differentiate the really awful things from the merely bad. I think that’s a fair summation, but you’re still better off clicking the link.
He hints at other reasons and I’d like to expand on one that’s near and dear to my heart: When we use “Hitler” or “Nazi” as a blanket term to describe anything we don’t like, we gloss over the things that made Hitler and the Nazis evil. They become cartoon characters, a lazy shorthand for “bad” without any understanding of the actions that made references to those names so heavy with meaning. Unless you’re talking about genocide, don’t compare someone or something to Hitler. Not only does it undermine your own argument, it makes light of what true evil is.