There were valid reasons for Mike Godwin to coin Godwin's Law. Nazi analogies have often been overused in the past. They are often used in frivolous ways or inappropriate circumstances. Very often, appeals to Nazism or fascism are indeed disrespectful to the victims of fascism and Nazism.
In my opinion, Godwin's Law has ceased to have any practical use, and is now more of an impediment to principled conversation. Fascism happened. Nazism happened. Their history exists, and like all history it is an appropriate subject for discussion and comparison, in order to better understand contemporary times. No, I don't like offhand comparisons to Nazis, nor do I want people to feel comfortable using "Nazi" as a catch-all insult. But we've gotten to a place where many feel unable to use discussions of Nazism and fascism in discussions where it is absurd not to. It is not, in fact, wholly unserious to talk about a political phenomenon which drove the world to the massive war in history and to the brink of a wholesale change in the global geopolitical situation. It is not necessarily untoward to compare contemporary events to a period when a race of people were brought to the brink of extermination. It's particularly wrong to deny making these comparisons as the phenomena in question are less than a hundred years old.
I think a lot of what animates the notion that discussions of Nazism are inherently off the table is the assumption that fascism can never happen again, or happen here. Nazism and fascism are imagined to be as fantastic and unlikely as a return to witch trials. That's an incredibly dangerous attitude. Fascism happened, it happened in some of the most advanced and prosperous nations on earth, and it can happen again. Salazar was in power in Portugal until 1968, in a modernized, Western European nation. There is a great danger in imagining that Adolf Hitler was some demon, some inhuman monster, rather than being an evil man. Monsters and demons don't have to be feared because they are not real. Evil men have never left us, and the notion that the United States is simply too pure or too good to ever fall under the sway of one is only a pleasant fantasy.
It discourages me, when I read discussions of various world leaders and strong men, that comparisons to Nazis appear to be simply not permissible. Some of these guys are fascists, or neo-fascists, or proto-fascists. No comparison, no analogy, is ever perfectly complete. It's up to the person making the comparison to describe exactly in which sense the two things are meant to be similar. If the person makes a comparison that is not serious or not valid, say so. But taking one of the most important and dangerous socio-political ideas of the 20th century and declaring it permanently out of bounds of rational discussion does no favors for any of us.
Update: OK, the problem is Dodd's Corollary, not Godwin's Law.