Syntax and grammatical rules are in flux and debatable. Below are a few examples.
First, some background:
The comma as we know it was invented by Aldo Manuzio, a printer working in Venice, circa 1500. It was intended to prevent confusion by separating things. In the Greek, komma means “something cut off,” a segment. (Aldo was printing Greek classics during the High Renaissance. The comma was a Renaissance invention.) As the comma proliferated, it started generating confusion. Basically, there are two schools of thought: One plays by ear, using the comma to mark a pause, like dynamics in music; if you were reading aloud, the comma would suggest when to take a breath. The other uses punctuation to clarify the meaning of a sentence by illuminating its underlying structure. Each school believes that the other gets carried away. It can be tense and kind of silly, like the argument among theologians about how many angels can fit on the head of a pin.
Written by Mary Norris, found in the most recent issue of The New Yorker. You can read more here.
Second, this is a classic image illustrating the debate over the Oxford comma (or serial comma).
Lastly, here is a snippet from an interview with experimental psychologist/linguist Steven Pinker, from December 30th 2014 in The Atlantic.
Porch: Does the comma go inside the closed quotation mark or outside?
Pinker: If I ruled the world, it would go outside.
Porch: That’s terrible. It looks terrible!
Pinker: Our British cousins don’t find it that ugly.
Porch: It looks untidy. It looks like a bedroom with clothes all over the floor.
Pinker: Your aesthetics may have been shaped by a lifetime of seeing it in the American pattern, but this would be a case in which any aesthetic reaction should be trumped by logic. Messing up the order of delimiters in a way that doesn’t reflect the logical nesting of their content is just an affront to an orderly mind.
Read more of the interview here.