I’m catching up on my Grammar Girl, and she has an insightful history of ain’t. Lexicide.com is all about words that die because they lose their unique definitions for incorrect, redundant ones. Ain’t is a word that fell out of favor due to racism, classism, and general snootiness. Mr. Whitman and Ms. Fogarty lament:
The stigmatization of ain’t is a pity, because without ain’t, there’s a gap in our system of contractions. When you negate the present tense of be and your subject is a pronoun, you usually have a choice between contracting the pronoun and the verb or the verb and the negative word. For example, you can write we’re not or we aren’t, they’re not or they aren’t, and you’re not or you aren’t. The lone exception is I, where your only choice in standard English is I’m not.
Just like split infinitives and ending a sentence with a preposition, there ain’t no good reason for shooing ain’t off to the side.