Makes you wonder what the Elizabethans did, before the rules of punctuation settled themselves.

Just finished a book, Alphabet to Email: How Written English Evolved and Where It's Heading, by Naomi S. Baron, that covers questions like this. The short answer is that the rules of punctuation still haven't settled themselves. There are basically two schools of thought about punctuation. The grammatical and the rhetorical. The rhetorical, punctuate to indicate the breaths taken while reading aloud, came first since the primary purpose of punctuation was to aid in the out loud reading of texts. More later, I gotta go.