Okay. So many people of our age, particularly, believe that commas and grammatical correctness in general is just one more way "the man" controls our writing. There are more reasons than the Harbrace, however, to use correct grammar and punctuation; there are also rhetorical choices we can make about our punctuation. Often, there isn't a right or a wrong way to punctuate a sentence. You'll choose your punctuation marks according to the effect you want to have on your argument or in your writing. I'm going to just dash off a couple of quotes from a David Sedaris essay we won't be reading in order to illustrate my point.

"The upside to being raised by what were essentially a pair of housecats was that we never had any enforced bedtime. At two a.m. on a school night, my mother would not say, "Go to sleep," but rather, "Shouldn't you be tired?" It wasn't a command but a sincere question, the answer provoking little more than a shrug. "Suit yourself," she'd say, pouring her thirtieth or forty-second cup of coffee. "I'm not sleepy either. Don't know why, but I'm not."

Every night was basically a slumber party, so when the real thing came along, my sister and I failed to show much of an interest. "But...we get to stay up as late as we want?" the hosts would say.

"And...?"

[...]

Aside from myself, there were three other guests at Walt's slumber party. None of them was particularly popular: they weren't good-looking enough for that--but each could hold his own on a playing field or in a discussion about cars."

From "Full House," by David Sedaris, taken from The Best American Nonrequired Reading 2004

textbook