Hacker School has a set of "social rules [...] designed to curtail specific behavior we've found to be destructive to a supportive, productive, and fun learning environment." One of them is "no feigning surprise":

The first rule means you shouldn't act surprised when people say they don't know something. This applies to both technical things ("What?! I can't believe you don't know what the stack is!") and non-technical things ("You don't know who RMS is?!"). Feigning surprise has absolutely no social or educational benefit: When people feign surprise, it's usually to make them feel better about themselves and others feel worse. And even when that's not the intention, it's almost always the effect. As you've probably already guessed, this rule is tightly coupled to our belief in the importance of people feeling comfortable saying "I don't know" and "I don't understand."

I think this is a good rule and when I find out someone doesn't know something that I think they "should" already know, I instead try to react as in xkcd 1053 (or by chalking it up to a momentary maladaptive brain activity change on their part, or by admitting that it's probably not that important that they know this thing). But I think "feigning surprise" is a bad name, because when I'm in this situation, I'm never pretending to be surprised in order to demonstrate how smart I am, I am always genuinely surprised. (Surprise means my model of the world is about to get better. Yay!)

I dunno. I feel like "I don't understand how anyone could believe X" is a much, much better position to take on issues than "I know exactly why my opponents disagree with me! It is because they are stupid and evil!" The former at least opens the possibility that your opponents believe things for good reasons that you don't understand -- which is often true!

I am imagining the following exchange:

"I don't understand how anyone could believe X!"

"Great, the first step to understanding is noticing that you don't understand. Now, let me show you why X is true..."

I suspect that most people saying the first line would not take well to hearing the second.

We could charitably translate "I don't understand how anyone could X" as "I notice that my model of people who X is so bad, that if I tried to explain it, I would probably generate a strawman".