It is my understanding that the contraction y’all was considered correct American English in times past. At what point was this word removed from valid American English?
An argument for the superiority of "y'all" over "you guys," which fills the same grammatical niche in other parts of the US that "y'all" does in the South.
Economical: one word, one syllable.
Gracefully takes a possessive. "Is that you guys's new minivan?" vs. "Is that y'all's new Tesla roadster?"
Y'all is actually never correct - especially in written English.
Above, several people have noted there is an absent second-person plural pronoun in English. This is untrue. Thee/thou/thy are the second person pronouns in English, singular and plural; you is our third person pronoun, singular and plural (ye is the antiquated plural). It is not unusual for pronouns in languages related to English to be the same for singular and plural forms, typically we are changing the verbs to indicate singularity and plurality. Thee/thou/thy/thine sound quite antiquated in English today. This phenomenon of the second person becoming archaic-sounding has happened with Romance languages as well, especially in Mexico. In Spanish, this is the vosotros form which is not heard often in Mexico, in late antiquity, second person plural also became less common in the Latin language. It is quite typical for languages to behave this way over time. Y'all is a stand-in for words that people generally feel are uncomfortable to say or they lack other words in their lexicon to get the meaning across. In absolutely every case, the word "you," can replace y'all. This happens in several other instances in English - you guys, you lot, and other stand-ins that are two words are grammatically correct, although that is not the most efficient use of the language and they should be avoided. Dang, darn, friggin', etc. are other stand-ins for words that people feel uncomfortable saying in company - the y'all problem is much more closely related to people feeling silly for saying old words (just as they do for avoiding profanity) than it is to there being a problem with the language. It could also be said it is a lack of education about the language; that is not judgmental, it is just true.