Didactic stories should be simple, clear, and short. Think Aesop’s fables. If they become more complex, at some point they become case studies. (Case studies are good to use, but it’s a separate question.)
Why repeat them? I don’t know Pearl’s book, so I am not familiar with his purposes in repeating stories. So I’ll offer reasons why I sometimes repeat them.
First let’s note that short stories and fables themselves often repeat a story within the tale, usually the magic number of three times, each time with variation. Repetition makes the story (or the lesson) easier to remember. The variations advance our understanding of the problem presented in the story, usually with the third time being the charm. And repetition makes the lesson easier to remember.
A story that teaches a fundamental principle might be repeated as often as the fundamental principle is applied. It being a fundamental principle, it will probably be applied relatively often in different contexts. Learning to apply it in such contexts must be an important goal in the course. Richard Hamming in “The Art of Doing Science and Engineering: Learning How to Learn” tells a story of two people, one who makes a random walk and one whose directions of motion somewhat line up. The first makes progress at an average rate proportional to $\sqrt{n}$ and the progress of the second will be proportional to $n$. Over time, the second person will outstrip the first. He revisits this story several times in examining examples of engineers who had successful careers and those who had stunted careers. It is one of the fundamental points of his course: Making decisions according to your career goals will advance your career more successfully than making them without any direction in mind. It must be effective because I read that book over fifteen years ago. It’s also a geeky parable, which for his audience and for me makes it easier to remember.
The following reason for repetition probably is not what Pearl does. Here variation is used in the repetitions to illustrate significant differences when elements are changed. I don’t really know a “story” per se that does this, but I can think of calculus examples. For instance $(\sin x)/x$ has three types of limits, but some students memorize, literally, “the limit of $\sin x$ over $x$ is $1$.” And they may very well write that no matter whether $x$ is approaching zero, infinity, or a nonzero number. @Carter’s answers also points out the importance of variation to help keep students from associating irrelevant elements of context to an example and from ignoring important elements.
Now for a reason not to repeat a story. A story that can be repeated multiple times, each time to illustrate a different fundamental principle, is probably too complicated to tell even once. Without a specific example, I might be going out on a limb. They apparently retold the Iliad many, many times in ancient Greece, and Aristotle draws several different lessons from it. It would be hard to do that in a semester-long class, though.
