I’m fascinated by the various dialects of English and have studied them a little in graduate school. I’ve defended the regional use of “you all” or “y’all” as serving a valuable grammatical purpose. Most languages have a plural form of the second person pronoun.
Actually “you” IS the plural form, which explains why it always takes a plural verb: “you ARE,” like “they are”; “you eat,” like “they eat,” but “he eats.” What happened is that we lost the second person SINGULAR form, which was “thou.” As in other European languages, the second person singular–Spanish “tu,” German “du,” English,”thou”–acquired also a social meaning, so that it began to be applied as a “familiar” pronoun, reserved for either social inferiors or to people you are very close to. That Luther in his vernacular translation of the Bible used “du” to refer to God–something echoed in the English Bible’s “thou”–meant that though He is the high King of heaven and earth, He is also our close, intimate, heavenly father.
Anyway, if we can’t bring the singular “thou” back (with its conjugations, “thee” and “thy”), we could at least make room for a new plural. In American English, the South has “you all”; some Northern dialects make the pronoun plural the same way we make nouns plural, by adding an “s,” resulting in “youse.” In some Southern dialects–I have heard it in Arkansas and Texas–there is the contraction of “you ones”: “you’uns.”
Last week, though, I heard “you all” used as a SINGULAR! A fellow-Virginian passed me in the parking lot and greeted me with “How are y’all?” Now that is incorrect! It’s using a plural for a singular! When we speak improperly–in the sense of using a non-standard dialect–we must be sure to do so correctly!