We are born understanding the grammar....in the vernacular, language is at its wildest and most dynamic.
In the vernacular, wishful bollocks!
Yes, linguistic diversity exists, but most languages have two forms; the formal and the vernacular. Welsh is a prime example, with written, bardic Welsh being very different from spoken Welsh.
In normal social intercourse the vernacular is fine and is capable of wonderful colour and clarity. There are areas, however, where 'grammatical correctness' is important. If you wish to communicate with someone outside your argot, then Standard English is the best medium. If you wish to convey subtlety, shade and nuance to an unseen audience, then Standard English is the best medium. If you wish to state something without fear of misunderstanding or equivocation (as in a court of law or in business) then Standard English is the best medium.
And, with some people, even outside formal intercourse the vernacular is deficient. I have encountered too many young people for whom inarticulacy has become a curse. They can express themselves to their peers, but they struggle to make themselves understood to anyone else. An inadequate vocabulary and a tenuous grasp of syntax stands in the way of communication and leads to frustration, anger and aggression. To airily claim that grammar is 'hard wired' into their brains is a facile, middle-class condescension.
So I will stick to my prejudice. I have neither the time nor the patience to teach the rudiments of Standard English to someone in my employ, and many younger people would regard any attempt on my part as insulting and patronising. Accordingly, I prefer to offer work to people with whom I can communicate easily and comfortably and who, I feel, can deal any client, from a duke to a dustman, with confidence and clarity. The ability to make good tea, tolerate Radio 3 and 4 and appreciate terrible puns would be nice, too.