Pity the reputation of the poor little "ain't." It's been around since sometime in the 1700s, showing up in the speech of just about everyone in England. Rich, poor or middle class, it found a home in the language usage of just about the entire population in that great nation.
But then something happened sometime in late 1800s. Rich and poor still made use of it, but the middle class took a disliking to it. I have no idea just why this happened, but once the idea took hold it wouldn't let go. Then in the early 1900s the rich decided they could do without it as well. Then the evil eye was cast upon it and it became a castaway in proper usage.
However, it could not be entirely discarded. Sometimes there's just no better way of putting a little emphasis to what you're saying. Consider the everyday saying, "If it ain't broke, don't fix it." Also "This ain't my first rodeo." Don't forget "You ain't seen nothing yet." And as Will Rogers said, "Common sense ain't common."
And where would songwriters be without it? Louis Jordon sang "There ain't nobody here but us chickens." One of Fats Domino's songs, "Ain't That a Shame" spoke to a whole lot of folks. Let's not forget the Ira and George Gershwin musical Porgy and Bess that gave us the classic, "Ain't Necessarily So." Another favorite was "Ain't No Mountain High Enough" by Nickolas Ashford and Valeria Simpson.
Dizzy Dean, Baseball Hall of Famer and announcer, summed it up when he said, "A lot of people who don't say ain't, ain't eating."
I'll just close by relating something said to be a common Texas saying (though I'd not heard it before) that tickled my fancy: If that ain't a fact, God's a possum.