I have no idea why I shouldn't write "all the cases". What are you trying to say?
* I understand all languages. -- unbounded set, applies to every language, everywhere
* I understand all the languages. -- bounded set, applies to a known set of languages, i.e. specifically those languages which are supported by a particular program
To be fair, I think "all cases" in this situation is also acceptable.
- The rules of a/the are simple.
- Not really, if you think so, then you haven't considered _all cases_. There's this exception and there's that exception. [...]
but:
- The rules of a/the are simple.
- Not really. There's this exception and there's that exception. [...] If you think the rules are simple, then you haven't considered _all the cases_.
[1] but take what I write with a big grain of salt, I can't explain why I wrote "the idea" or "the start" and I'm not 100% sure if that's correct :)
- I don't think the explanation of "all cases" vs "all the cases" fully works. I agree with other commenters that in this example, either phrase might be used. (Whether the sentences comes after a point-by-point discussion or before.)
To me, the version without the has a less natural feeling. You might describe it as more formal, or more stilted, or more abrupt, but the main point is that it's going to be somewhat dispreferred. There isn't necessarily a reason for the preference to tilt one way or the other.