I can't not agree with you lol.

I majored in English and I have studied a few other languages (sadly, I never could learn to speak them well) and I noticed spoken language is a lot more fluid. Saying "I ain't got nothing" wouldn't fly in college paper but, like your example, no one who knows the language would be confused if someone said it.

"After all, educated people tend to know when to turn the switch off, when it's time to speak properly and when they can relax and be a little silly. "

I agree with this. I think some of it also aids in communication. If I went around talking like I would in a class with my stoner frineds they would laugh at me and, while they would underatand me, they would think that I am being strange for being unnecessarily formal. That being the case, I don't say, "hello good sirs, can I interest you in partaking in this cannabis cigarette?" Instead, I might say "hey, you guys want to burn this doobie?" That is not exactly accurate. I am not sure that "doobie" is even a real noun and describing smoking it as "burning" it isn't the best way to phrase the act but makeing the statement in that way feels more natural for the situation.