Like it or not, “ain’t” is a word. Etymologists treat it as a word; historically it’s the contraction of “am not.” It functions as a word in speech. Yes, you’re unlikely to find “ain’t” in written English, but you will find it in spoken English, but that doesn’t make it any less of a word.
Fundamentally, one must ask, What is a word? A word is basically a communicative symbol. If using “ain’t” makes one understood, expresses one’s thoughts to others, then it has achieved its purpose as a word. “Ain’t” might not be the most precise, most proper word, but if its use helps to put ones thoughts across to others then it has all the attributes of being a word.
In some parts of the United States, “y’all” is a very proper word. Hell, I use “y’all,” because it conveys a very precise meaning. The development of English left the second person plural without a pronoun; “y’all” fills that grammatical “hole.” Living in Pennsylvania I sometimes get odd looks when I speak and out comes “y’all” because it’s not “proper” English, and I would never write “y’all” in a sentence. But it has its time and its place, and it conveys its own meaning, a precise meaning.
That’s what all language is about, the desire to be understood by others. The use of words facilitates that, using symbols to describe in shorthand abstract concepts. Many of the language “rules” that are cited were designed in the 1800s as a way to enforce class distinctions; those using the “rules” would mark themselves as part of the aristocracy/upper class, while those who consistently “broke the rules” would, by their language use, show themselves to be part of the lower classes. That debate continues to today; the issue of ebonics a decade ago has its roots in the marking of class distinctions based upon word and grammar usage.