Ain’t was always a valid contraction for “am not”, but too many people misused it until grammarians convinced everyone it wasn’t a real word. Besides, words aren’t handed down from on high. If it’s in the dictionary it’s because it was already in use. What’s the point of a dictionary if you can’t look up the words people are using?
In the 70s and 80s we were taught that it wasn’t a proper word. Of course, whether it’s proper or not, I hate it.
I also hate “irregardless”. If I said that word to my English teacher I would have been slapped into next Tuesday.