The only style manual/grammar resource where I've ever heard such practice being acceptable is the Associated Press one, which omits them due to space constraints. All American authorities: The Chicago Manual of Style, Elements of Style, The Gregg Reference Manual, Oxford University Press, etc. say to use the serial comma because it reduces ambiguity.
So why aren't people using it?
Well, at least I'm not the only one who finds this tendency aggravating.
Yes, I'm a grammar geek. Ye verily, I have embraced my nerditude.
*I'm less peeved by Brit and Aussie writers who do it because their style manuals waffle on it more, and they've got other things to worry about, like whether to put the punctuation on the inside or the outside of the quotation marks.
I got a piece of mail from the excellent folks at Cricket that perked me up, although when I first opened it, I was momentarily confused. The communique inside was addressed, "Dear Pussywillow and Ladybug"--which are not names I recall going by--and there wasn't an explanatory note to aid my comprehension. But after a moment, a few more neurons roused themselves, and I realized "Pussywillow" and "Ladybug" were the names of a couple of the bug characters featured in Cricket. It then dawned on me that they'd printed and sent me an email they'd received from a little girl, Maddy age 11. She loved the April 2006 issue and specifically mentioned my story, "The King of Rabbits and Moon Lake," as one that she enjoyed
Fan mail from a young reader! I'm tickled that a little girl was so enthusiastic about reading, and specifically about reading something I wrote, that she took the time to send a letter to the editors. Happy writer me.