The serial comma, aka the Oxford comma, is the toilet paper roll of writing style dicta, for like the toilet paper, it gets people tied in knots when it doesn't go their way.
Its presence or absence is also just about as important as which way the toilet paper is hung. There are reasonable arguments to be made to include or omit it, but either way, your meaning will probably get across, except in a few odd circumstances (some in favor of the comma, others against it) that punctuation purists on either side point to as evidence that the language gods are with them.
This particular comma even shows up in online dating profiles. Even those who miss the distinction between you're and your feel confident expressing a strong opinion about this small mark and using it as a gauge of potential partners' worthiness. Maybe there's something deep here that I'm not getting, some direct correlation between the use or non-use of this comma and the potential for a bad breakup when one partner receives an inappropriately punctuated text message from the other. Or maybe this kind of pseudo-intellectual posturing is a useful tool for flirting.
But even though I do have an opinion on this issue (and it is strongly held), I have to believe that those who are completely hung up on the distinction between a sentence that uses commas before all elements of a list (a, b, and c) and one that omits the final one before the coordinating conjunction (a, b and c) deserve a limited slate of romantic options.
People who don't like the serial comma may bring up the possibility that it may be perceived in some circumstances as the final comma setting off an appositive:
Rick, my brother, and I threw eggs at my neighbor's expensive stucco wall.
Take out that comma, and it's less likely that readers will think Rick is the narrator's brother:
Rick, my brother and I threw eggs at my neighbor's expensive stucco wall.
Those who do like it offer examples in which the comma's absence tends to blur necessary distinctions:
The plaintiff's lawyer waved an arm at the offenders, the defense lawyer and the judge.
It could be argued that in a situation like this, the last two items in the series might be considered an appositive defining the offenders as the defense lawyer and the judge. With the serial comma, the distinction is clear:
The plaintiff's lawyer waved an arm at the offenders, the defense lawyer, and the judge.
Whether a reader would actually misunderstand the first example in either of these instances depends on the context and on the reader's expectations. A careful writer would find a way to reorder the items or recast the sentence in a case in which extreme confusion was a probable outcome.
In this blog, I have chosen to use the serial comma, though the stylebook I use at the office prefers to omit it. My own reason for liking it is about aesthetics more than clarity: A brief pause before the end of the series lends the sentence an air of formality. It also gives the reader a chance to take a small breath before lunging toward the last item in the list. It's like a vacation that only lasts for an instant in time that is still vaguely perceptible, even though its duration is infinitesimal.
That's not a scientific explanation or much of an argument, but it's as reasonable and definitive as anything that might be said about how to hang the toilet paper.
Bonus opinion: This type of comma may be called the serial, Oxford, or Harvard comma. I prefer the more generic term "serial comma," both because it describes how it is used (in a series) and because this usage was not invented at either Oxford or Harvard. Also, pledging allegiance to an "Oxford comma" sounds like a snooty attempt at name-dropping, especially if you're using it to bait the hook for a potential date.