However, one thing that has always baffled me is why fiction publishers use manuals of style for copy editing manuscripts. In my case, per the abbreviated notation in the style sheet that accompanied my copy edit, M-W 11th, Chicago 15th, Words into Type, and Garner’s Modern American Usage.
I do understand why some aspects of house style are important, such as getting the ellipses and em dashes correct. That's a book design and typesetting thing. For example, the style sheet says the following:
“Use this form—” When an action. “—interrupts the speech.”
“Use this form”—when an action occurs simultaneous to speech—“without interrupting it.”
Okay. Fine with me. This is how Tor wants their books to look. Hooray! I'm not a book designer, and I certainly didn't embed any punctuation geekery in the manuscript I turned into them.
But on usage and spelling...? Fiction is in one important sense all about voice. And there's a lot of changes that get made in the copy edit that I have to stet. There are certain archaic or non-standard spellings I favor. "Storey" for "story" when describing buildings. "Dreamt" instead of "dreamed". "Til" instead of "till". All of which get carefully amended to the current standard written usage, and all of which I just as carefully stet back to my original.
Don't even get me started on the that/which distinction. The rule about restrictive and non-restrictive clauses is a piece of prescriptivism demonstrably at odds with the way people actually use those words, and I personally will deliberately stray from the rule for the sake of smoothness of the reading. (i.e., not creating a clunky string of serial uses of "that" or "which")
Likewise "who" and "whom". I know the difference perfectly well, thank you. But almost no one uses "whom" in casual speech, so in dialog my characters don't, unless they're the sort of personality who would be either that formal or that persnickety. Also, "they/their" for third person gender indeterminate is a very common usage dating back hundreds of years in English, and really doesn't need to be corrected.
Oh, and comma splices, I loves me some comma splices when I'm writing fiction. So what? It's my voice.
Fiction isn't formally correct, and it shouldn't be. It should reflect the author's voice. I can write very formally when I need to. I do it all the time for business writing in the Day Jobbe (though that has its own usages and quirks). I also do some legal writing in the Day Jobbe (disclaimer: I am not an attorney and I do not practice law, I do, however, routinely draft certain contract provisions for our Legal department to review), as well as some technical writing that is distinct from my business writing. I even occasionally do marketing writing there, though less often than I used to. Each of those forms has their distinct speech register, expected norms of usage, and formalisms.
The really great thing about fiction is that you get to craft your own speech registers, your own norms of usage, and your own formalisms. While I definitely need to be internally consistent in style and usage within the text (though I can readily imagine exceptions even to that statement), I don't need to be consistent to formal usage, so long as I remain clear and comprehensible.
So I'm always puzzled about why publishers instruct copy editors to round off all the interesting bits.