Y’all ought to come down to New Orleans sometime, baby.
Shouldn't that be "all y'all" when using proper Southern parlance?
"All y'all" would mostly only be used in situations when it needs to be stressed that the statement/question applies to every member of a group individually, as opposed to only applying to the group in a general or collective sense.
For example: "Do y'all need an extra helping of banana pudding?" carries a slightly different meaning than "Do all y'all need an extra helping of banana pudding?"Ah, I see. I've been aware of the distinction in the South, but I wasn't aware of the distinction at that level. From how it works, "all y'all" still would work in the sentence we're talking about, right?
"Y'all" has definitely become more widespread than just the South, but I've never heard "all y'all" outside of a Southern context.