Call me a grammar snob, but I cringe just reading "I seen". I feel like I have to flush my eyes with Visine now! And I can't hear "youse" without an image of Bobby De Niro popping into my head.
Y'all is different, since as others have noted, English lacks a distinction between "you" as a single or a plural form. So it's simply a contraction of "you all", which specifies that the speaker is communicating to a group.
But I am dead set against allowing the vernacular to be taught in schools, or permitted in verbal and written work in the classroom. (It calls to mind the debate over Ebonics in the 1990s, in which some educators sought to incorporate so-called black slang into the curriculum.)
Eilonwy is accurate (as usual) about the profound influence of the Scots-Irish on American society, particularly in the South. I just started reading Born Fighting, which tells this story in a very compelling way. (The author, Jim Webb, is a bit of a misogynist, having provoked controversy with his essay "Women Can't Fight", but that's a different topic altogether!)