The Economist explains
SEVERAL months ago I was surprised to see Arnold Zwicky, a linguist, use a comma splice. A few commenters took me to task for being over-picky. The question came up again in the comments several days ago, when k.a.gardner, a frequent commenter, asked for a post on the comma splice. One of my colleagues quickly replied that "The comma-splice rule is totally arbitrary," and a back-and-forth ensued.
What is a comma splice? Prof Zwicky wrote back in July
"this is not even a tempest in a teapot, it's a fuss in a thimbleful of spit."
That's two independent clauses joined only by a comma, or a comma splice, sometimes called a "comma fault". Ashbird, another commenter, was taught as I was: the comma-splice is an error. My senior English teacher marked down any paper with even a single comma splice by two letter grades, so that an otherwise perfect A paper would receive a C. (She applied the same rule for fragments and run-on sentences.) My colleague, however, says it's a matter of style. The Economist doesn't have a ruling on comma splices in the style book, but I don't recall ever having seen one in the newspaper. And I would notice; my English teacher's injunction gave me a terror-loathing of comma splices that has never left me.
But, as ever, there are facts to be had, and in cases like this, the go-to reference is the Merriam-Webster Dictionary of English Usage. (It is hard to praise this book enough.) Sure enough, the comma splice was once part of the best English usage:
As to the old one, I knew not what to do with him, he was so fierce I durst not go into the pit to him — (Daniel Defoe, Robinson Crusoe, 1719)
The New Jersey job was obtained, I contrived a copperplate press for it — (Benjamin Franklin, Autobiography, 1771)
By the nineteenth century, MWDEU gives examples by Lord Byron, Jane Austin and Lewis Carroll, but only in personal letters. This trend continued into the 20th century, where Ronald Reagan and E.B. White both used comma splices in letters. (Yes, that is the E.B. White who put his name, in "The Elements of Style", to this crisp injunction: "Do not join independent clauses with a comma," though "Elements" allows for rare exceptions if the sentences are very short and closely related.)
By the 20th century, rulebooks commonly warned against the comma splice, with the effect that it now seems limited to informal writing such as letters, or in reported speech.
The Ambassador...responded with a blast of enthusiasm. "Those weren't tough questions, those were kid-glove questions..." — John Updike, Bech is Back, 1982
MWDEU, which often debunks sticklerish rules with massed evidence from indisputably great writers, says "uncorrected examples are so hard to find in print" that "You should not try the device [of the comma splice] unless you are very sure of what you want it to accomplish."
I agree. Editors have made the comma splice so rare that they leap off the page (unpleasantly so, for me) when I spot one. The comma splice is unnecessary; a brief pause between two related thoughts can be accomplished by a semicolon like the one in this sentence. A full stop separates two thoughts more cleanly. Unless you're being aphoristic ("Man proposes, God disposes") or intentionally seeking a loose-knit style, beware that a comma splice is probably not worth the readers it will irritate.
Update: Stan Carey, in a thoughtful post with lots of evidence, was softer on the comma splice than I am. He also reminds me that "Elements of Style" allows for limited comma splicing, so I added a bit to this effect above.