Isn't it also the worst argument :-) In that it is well explained but I look a little bit in horror on it.

For me it seems like dynamic scoping is very similar or at least related to dependency injection. Which seems to be one of those things that really have polarized opinions of developers. Some people love it because the code looks really neat and it is really easy to add new pieces. Other hates it because it is really hard to understand what is happening under the hood.

I would say that dynamic scoping is a bit worse (in my opinion) in that it is like dependency injection that are supposed to be modified one or more times in between so it will be almost impossible to figure out why a value is what it is when it goes wrong.

Incidentally that makes it super easy to mock out or intercept subsystems if they use dynamically scoped hooks e.g. in most languages intercepting stdio is a pain in the ass (you might have to swap out multiple globals, it's not thread-safe, etc…).

If you're in CL however, you can just rebind standard-output and you'll capture what anyone downstack sends there.