alt Hacker NewsMeaningless toy examples have the problem that they can appear to make sense, planting the seeds of terrible ideas.
If in a course example Dog extends Animal, it can be an arbitrary demonstration of language mechanisms (with an uncontroversial is-a relationship) but even in that case it is implicitly suggested that it is a good or "normal" design, implying an alarmingly complex program that has good reasons to deal with those two types.
Such a program is usually not described for brevity, giving the false impression that it exists: if the problem were analyzed with any diligence, usually Dog would appear a completely pointless complication.
More or less, this is the https://en.wikipedia.org/wiki/Lie-to-children debate. It'd be nice to always be able to learn the best known things up front, it's not usually a particularly practical approach to learning a complex field.
I.e. planting a terrible idea is alright so long as by the end the terrible idea was able to be replaced down the line in less time than trying to learn everything "correctly" from the get go. The latter part is where I felt the class failed, it held on to bad idea through the end instead of quickly replacing it with the "next level" of conceptual thinking.