As I've argued before, Nazi comparisons are utterly shameless and despicable; the latest being Erick Erickson's reference to Whitehouse Healthcare Spokeswoman Linda Douglas as "the Joseph Goebbels of the White House Health Care shop."
But let's see, what is it about Goebbels that makes him so memorable as a propagandist. Is it because he invented propaganda? No, surely propaganda has been used for thousands of years. Is it because he was a propagandist and propaganda is always wrong? No, America has used propaganda, including against Goebbels' government in WWII and that is considered a good thing. Could it be because Goebbels built the best propaganda machine ever? No, I would think the Soviets actually did a better job at that, even if I can't name any of their propaganda guys off-hand.
No, the reason Goebbels is so famous is probably because of the whole Nazi, Jew-hating thing; with the burning of books, and the killing of Jews, and the things of that nature. And the reason to reference him isn't to reference a master propagandist, but an evil one. Just as I argued before, the reason to evoke the Holocaust is to make a Nazi connection. That's the whole point. That's why you do it. And if you're not trying to make a Nazi connection, then you're just lying.
Propagandist by Profession
And what's stupid about Erickson's reference is that, to call a spokesperson a "propagandist" is absurdist, as that's the nature of the job. A spokesperson's job isn't to be an objective reporter of reality, but to give the best spin of their employer's side of things. And that's taken as given. Nobody assumes that Linda Douglas is going to say negative things about Obama's heatlhcare plan, and if she did, I'd expect her to be fired. But is she burning books, destroying businesses, and sending conservatives to concentration camps? Not that I've heard of, and Erickson hasn't suggested it's happening either.
And those are the things that make Goebbels memorable. Had he just been the Baghdad Bob of Nazi Germany and acted like a typical spinmeister, nobody would have remembered him. But he wasn't your standard propaganda machine. And just like the Holocaust should only be used to describe events that are comparable to the Holocaust (ie, millions of lives intentionally destroyed for an evil purpose), references to Goebbels should be limited to people who did what Goebbels did. And anyone who does otherwise lacks any credibility.
Now, if your purpose is to link Nazi atrocities to current atrocities, that could be acceptable; depending on the circumstance. But any professions of innocence, and that you're merely referencing Goebbels' propaganda techniques or the generic usage of the word Holocaust, are absolute frauds by people who lack intellectual honesty and are lying to themselves. Whether or not you agree with Linda Douglas, she'll never be Joseph Goebbels.
The Purpose of References
And just to clarify the point: The reason to evoke something is to reference what that person or thing is best remembered for. For example, while Baghdad Bob was also a propagandist for an evil dictator, he is best remembered as a comical character whose lies were betrayed by basic undeniable facts which exposed his absurd falsehoods. And if Erickson had compared Douglas to "Bob," nobody would assume it was because she was associated with rape rooms and torture; even if "Bob's" regime was famously associated with both.
References are used as quickie knowledge capsules, to convey a large amount of information in a short space. But it's not enough simply to find some attribute that is similar to the negative reference, if you're not explictly attempting to link the person to the primary attribute the person is known for. For example, linking liberals to Nazis just because they both have vegetarian members is utterly absurd, unless you were stupid enough to believe that Nazis are only famous because they didn't eat meat.
And the rule is that if the purpose of your reference isn't directly obvious to your audience, then your reference was a failure (Dennis Miller, I'm looking at you). And if the purpose of your reference isn't directly related to how that reference connects to your target (ie, comparing any propagandist you don't like to Joseph Goebbels), it is a fraud.
Overall, unless you're trying to evoke the specific reason for why something is famous (or infamous), you shouldn't use it. Abraham Lincoln isn't best known for being tall, nihilists aren't best known for being postmodernists, and Goebbels isn't best known for basic propaganda. And if you're evoking a BIG NEGATIVE without trying to link the BIG NEGATIVITY to the person you're targeting, then you're an outright liar who should avoid speaking until you get something better to say.