A sentence should contain no unnecessary commas for the same reason that a symphony should have no unnecessary pauses. True, commas add rhythm, and more importantly clarity, to our writing. But, if we use too many, of them, our writing becomes difficult, for people, to read, and our ideas end up fragmented, instead of connected.
For years participants in my grammar and writing workshops have magnanimously imparted their golden rule for commas: use a comma whenever you would take a breath. And for years I have regretfully but pointedly burst their bubble. That simple rule, which so many have clung to since their tender years, works occasionally (even often, if you’re a speechwriter or playwright), but it also gives rise to the superfluous commas that pollute our prose, bobbing up disconcertingly like plastic bottles in the ocean.