They are a vital element of effective teaching, especially because they are the only times when you can be highly confident your students are paying attention. But wandering takes a deep command of your subject matter, at least of the ideas being discussed, if not the details that make up the idea.
At Hope Academy in St. Paul one of the teachers described rabbit trails as “organic curiosities.” I like that. Another pointed out that the classical teacher can bring eagerness and understanding of metacognition to the the classroom discussion. I’ve been trying to figure that out ever since, but it sounds fantastic.