My opinion is y'all is dialect, not proper English. Yes, it could become proper, just as TV became for television or xerox for photocopy, but it takes more widespread adoption. I'm not even sure who gets to decide when something becomes proper English but I feel a bit stodgy about it. The "I learned it a certain way so why should it change" attitude.
I've mentioned the Chicago Manual of Style before along with Strunk & White. If something is clearly defined in those then that, to me, is the "right" way and the rest of you can get off my lawn. At least I know I'm peculiar about it.
