Jmacusa, you and I have exchanged very friendly posts concerning the loss of your cat, so I want to be nice. But that is patently ridiculous.
"Y'all" is a contraction of two words. It's no more slovenly than any other contraction, and is perfectly acceptable. What sets it apart is the fact that it's purely regional. If one has an objection to it, and not to the use of all other contractions, then it's fair to say the person's problem is with the region itself, which would not reflect well upon that individual.
Now, I could see having a problem with the word "ain't", which is ungrammatical (though I wouldn't go so far as to call it slovenly). I've seen you use "ain't" in your posts, which makes your objection to "y'all" even more mystifying.
For the record, I occasionally use "ain't" myself when in a private setting with family or friends.
Here's something interesting: