I liked Frailty. Without giving too much away, it demonstrates how any agent with god-like powers could make you believe whatever it wants you to believe. Therefore if you ever find yourself in a universe in which gods and powerful demons exist and have an interest in influencing your life, you could fall into a situation where you have no means of determining which gods or demons are best aligned with your long-term goals. Also, a sufficiently cogent propaganda campaign could have a good chance of convincing you to commit atrocities.
You are right. However, I don't think the advice was meant to be used to evaluate weather or not a given essay is boring. I found it helpful because it provided a simple rule that I used to change my writing habits. I had a vague sense that some of my sentences were stilted, but I didn't know how to remedy that problem. Not everyone will get the same utility from creative restrictions, but I find them inspiring.
Could you give an example of "three reasons why [x] is good, and one broader conceptual reason why it might not be”? I’m not sure I follow.