While I'm probably responding to a joke, intelligibility isn't simply a binary. It's how we're able to have interesting connections between dialects or Danish, Swedish, Norwegian for example.
The first rule of communication is knowing your audience and if one's language can be adapted in simplicity, complexity, lexicon, grammar, content, style, voice, medium &c to be more readily understood, then it's in the speaker's benefit to do so. If GP's teacher could not earnestly understand vulgar English then by all means adapt, but the sense that I'm getting is that they were being obstinate on purpose and those sort of people should live in the communicative bubble of isolation they put themselves in.
Well, my "joke" was ment as a demonstration, that the rules have the purpose to make it easy to be understood. I bent the rules a bit, so that it should still be possible to decode my message, but not easily. This should demonstrate that rules are not completely arbitrary, even if (some) people can understand what I am saying.
The first rule of communication is knowing your audience and if one's language can be adapted in simplicity, complexity, lexicon, grammar, content, style, voice, medium &c to be more readily understood, then it's in the speaker's benefit to do so. If GP's teacher could not earnestly understand vulgar English then by all means adapt, but the sense that I'm getting is that they were being obstinate on purpose and those sort of people should live in the communicative bubble of isolation they put themselves in.