There's another usage that comes to mind, though. One might argue that "y'all" borders on a second person plural inclusive of the speaker whereas "all y'all" marks a distinction between the speaker and the others. For instance, a peeved person would be more likely to say, "All y'all can kiss my ass," as opposed to, "Y'all can kiss my ass." "Y'all" by itself is more friendly and self-inclusive than "all y'all", which carries an inherent otherness to it.