It is a wonder we learned anything at all and it made English tests a hellish experience, since it was never clear what was actually correct. This was on top of English itself being a confusing language (see all of the words ending in ough if you need an example). English teachers seemed to assume we had some magical ability to know the right answer after confusing us with conflicting examples, and we rarely did. It is no wonder back then that they said the same things about us.
If they want to improve literacy, they should replace the required reading with books that have no flaws in the spelling and grammar to reinforce English lessons. Then we might see student literacy rise as students stop being so confused.
I learned English as a second language and while I did have formal lessons, the most bang for buck learning came from stuff like Huckleberry Finn and forums and random songs (especially hip hop) and cartoons. Not because they are "good english" but precisely because they are "bad english" but still capture the spirit of the language. The meaning is there and it's English as used by native speakers. i.e.: It helped me internalize the language. Maybe it was because I had no one around to practice informal English and I needed that and it's different for native speakers. Dunno.
I can't say it didn't confuse me but I know it didn't negatively impact my scores. Despite not doing my homework and not even having the required textbooks or studying properly, it was the subject I had the easiest time and best grades aside from math or programming. And it's not like the bar was low - we had to pass C1 on the CEFR and I've easily passed C2 and have almost 100% on TOEFL and similar.
Also I'm not particularly gifted with languages - I don't know grammar at all and I struggle speaking all the time even in my native language.
I do not speak that way and I do not know anyone else who speaks that way.
That said, being relatively average at English as a student because I was constantly confused by the literature I was forced to read did negatively affect my test scores.
"bad english" by expert native speakers is different than "bad english" from poor native speakers is different than "bad english" from still-learning non-speakers. IMO the first two both benefit your understanding of the language and help with reading comprehension because they expand your personal knowledge base of how many ways you can convey the same meaning in English with different words, order, grammar, etc and that is crucial for a flexible language like English.
But your reply reminded me of a friend that's very good with languages but only languages that have a rigid structure - he's from the UK but he struggles with English yet he's amazing in German and Chinese and several others I don't remember.