I think it's all just normal evolution of language. I actually think it's kinda neat how different English-speaking subcultures found different ways to reintroduce the pronoun.
> All y'all is used in the Southern United States when a speaker wishes to include everyone being addressed. Y'all may refer to an indefinite set of members of a group, but all y'all definitively includes everyone in the group.
Also, is it just me or does that Wikipedia entry have zero citations? I thought Wikipedia was allergic to citation-free claims..
I'm guessing it's regional and/or group specific.