In
Not with a Bang but a Whimper: The Politics and Culture of Decline, "The Gift of Language" Theodore Dalrymple describes what he has observed of the result of years of teaching English in a non-prescriptive fashion:
With a very limited vocabulary, it is impossible to make, or at least to express, important distinctions and to examine any question with conceptual care. My patients often had no words to describe what they were feeling, except in the crudest possible way, with expostulations, exclamations, and physical displays of emotion. Often, by guesswork and my experience of other patients, I could put things into words for them, words that they grasped at eagerly. Everything was on the tip of their tongue, rarely or never reaching the stage of expression out loud. They struggled even to describe in a consecutive and logical fashion what had happened to them, at least without a great deal of prompting. Complex narrative and most abstractions were closed to them.
In their dealings with authority, they were at a huge disadvantage—a disaster, since so many of them depended upon various public bureaucracies for so many of their needs, from their housing and health care to their income and the education of their children. I would find myself dealing on their behalf with those bureaucracies, which were often simultaneously bullying and incompetent; and what officialdom had claimed for months or even years to be impossible suddenly, on my intervention, became possible within a week. Of course it was not my mastery of language along that produced this result; rather my mastery of language signaled my capacity to make serious trouble for the bureaucrats if they did not do as I asked. I do not think it is a coincidence that the offices of all those bureaucracies were increasingly installing security barriers against the physical attacks on the staff by enraged but inarticulate dependents.
(...)
Beginning in the 1950s, Basil Bernstein, a London University researcher, demonstrated the difference between the speech of middle- and working-class children, controlling for whatever it is that IQ measures. Working-class speech, tethered closely to the here and now, lacked the very aspects of standard English needed to express abstract or general ideas and to place personal experience in temporal or any other perspective. Thus, unless Pinker's despised schoolmarms were to take the working-class children in hand and deliberately teach them another speech code, they were doomed to remain where they were, at the bottom of a society that was itself much the poorer for not taking full advantage of their abilities, and that indeed would pay a steep penalty for not doing so. An intelligent man who can make no constructive use of his intelligence is likely to make a destructive, and self-destructive, use of it.
We're collectively sort of shifting the discussion. I think it's possible to use slang terms
and have a rich vocabulary and the use of slang does not, in itself, mean that one is mimicking a person that has a poor vocabulary or glorifying those that do.
I started to really reflect on the issue you quoted a few years ago when I was living in Japan. I had never really considered the link between language and development until a friend of mine brought up a concern she had.
In Okinawa there's a school called Okinawa Christian School International (OCSI). When it started, decades ago, it served a valuable missionary purpose given the poverty of the island but now it's sort of supported by Westerners who sort of "pay in" to being a missionary and teach at the school and my friend, who had taught there, was concerned it was actually harming and not helping the students in the long run.
Words are really symbols for ideas and, as we progress educationally, we ought to be filling minds with more and more ideas that will have words to give them expression. Her concern was that OCSI neither teaches mostly Okinawan children enough English to convey enough of a vocabulary nor Japanese for the same purpose. In other words, if I am well trained in one language then I can take that knowledge of ideas that have words associate with them and bridge into another language that might have different words but the same basic colllection of ideas. The less vocabulary I have, the less ability I have to communicate propositions in any language. Because the students learn poor English and aren't in Japanese schools where they learn Japaneses, the students are impoverished by such an education.
Sadly, this is where a lot of people are in the United States. It is not merely a problem for the "ghetto" but many people have an undeveloped vocabulary and are therefore undeveloped intellectually. I found this particularly relevant as, in a recent discussion, it was pointed out how many concepts have come into our vocabulary from the Christian religion. There are ideas that are Scriptural that are nearly impossible to convey in other languages because, culturally, the language has been impoverished by idolatry.