It's not rational not judge sites by the content, rather than some 'mirror' issue.
I'm not judging their content based on "some 'mirror' issue". I judge them as mirror sites because of their content. Considering that you weren't even aware of the posting rules at DU, I don't see how you are any more qualified to judge the place than I am.
And the truth is in a moderate position, the world is halfway between.
The truth isn't always right smack in the middle, but it's usually closer to the middle than the far ends, yes.
For all the criticism of DU, not one word has specifically supported it with any credible review of the actual content. It's just assumed 'oh, it's a group, so they're all equally as wrong as any other group - no need to actually have any evidence (beyond maybe an outlier anecdote or two), just assume and say every group besides yours is equally bad'.
What's there to say about it? The problem is the attempt to label a site like that "good" or "bad". It's neither, it just is what it is. It's a partisan site filled with far-left opinions made by far-left people to other far-left people. They like it, good for them! I don't care. They've made it clear that I'm not welcome, so I don't go there, at least not usually.
Of course any site that is for Libertarians to discuss their views where it's not welcome to have others come and trash every thread attacking Libertarniasm is equally bad, right?
Can you name such a site? I doubt it.
Clearly, holocaust deniers and holocaust historians are equally bad - and if each had a site where the other isn't welcome that proves they are mirrors or each other, both bad.
You start with a site dedicated to left-wing ideology and a site dedicated to right-wing ideology, and then compare them to "holocaust historians" and "holocaust deniers". In so doing, you are implying that your particular ideology is historically factual while the other ideology is factually incorrect and even dishonest.
The fact that you would even do this demonstrates your own subjectivity. You think that your viewpoints represent objective truth and that the viewpoints of your enemies are comparable to Holocaust revisionism. That's why you try to decide if partisan sites are "good" or "bad" while everyone else realizes that when it comes to politics, "good" and "bad" are in the eye of the beholder.