Must writing always be done in the tone/style of ordinary contemporary speech?
For example, some people might say that the conjunction "for" is archaic and should not be used in modern prose, but it seems to be a part of modern English according to grammar guides and the dictionary. The argument seems to be that since no one talks like that, no one should write like that.
But is it wrong to use the conjunction "for"? Do people always have to write in the tone of ordinary speech?
If you are writing fiction you may use archaic speech to make a character unique, or you may have everyone speak that way to show how the society acts. Another option is trying to show the reader that they are in a time that is unlike the one they are used to. It is not required to write in the tone of ordinary speech if the character you are trying to portray is supposed to be different from what is perceived as "normal" or "ordinary". It can also be used as the voice for your narration. Archaic speech could show that you are aiming for a fairytale style.
If, on the other hand, you are not writing fiction, but for example a thesis, you may not want to use archaic speech as it would be a bit more difficult to understand and is quite often unnecessary, making it a distraction from the content your are trying to make the reader understand. Though many people believe that it can potentially make you sound smarter.
Of course in any case you as the author can decide whether you like it or not. If there is an editor involved he may change some things, but if you prefer to sound archaic for whatever reason then that is your choice. The normal argument is that things that are not "normal" are often harder to understand, which may or may not be what you want to achieve and which may or may not put your readers off, depending on who your target audience is, what your medium is, what content you are trying to show and what feelings you want to convey.
The conjunction 'for' has fallen out of favor, what modern people say is "because". However, if your character was raised in an isolated community or circumstance that continue to speak like people did a century or more ago, using "for" instead "because" along with other grammatical oddities could be a unique quirk of speech for them.
We'd still understand them, I'd only use them where they'd naturally appear, but it provides the occasional reminder they are unique. (Not so many quirks that the reminders are constant.) For example, I wrote a math wizard that never says the word "So", he says "Thus" or "Therefore", as we would in a formal proof. Even if he is explaining why pepperoni should only be on half the pizza.
You should never write in the tone of ordinary speech. Ordinary speech is unreadable. It is repetitive, broken, trivial, and largely mindless. Dialogue is not speech. Dialogue should be crisp, relevant, coherent, readable, and should move the story along.
As such, dialogue is always to a greater or lesser extend stylized. In many cases writers will stylize the speech of different characters in different ways in order to provide a clear differentiation of the speakers for the reader. (Does Sam talk like Gandalf, or Gollum like Gimli?)
Whether "for" as a conjunction is truly obsolete is debatable. Certainly "because" is the lazy alternative today, but for is by no means archaic. You could well use the choice of for rather then because to stylize on character's speech. (And "'cus", "on account of", "since", and "reason being" for other characters. Of such trivial devices, sometimes, are distinctive voices made.)
No one actually writes exactly as they speak, it would be practically unreadable with all the "ums" and pauses, and non verbal communication. So, what we put on the page is always a stand-in for the spoken word anyway. It's also the case that people speak in many different registers, so there's no one set "tone of ordinary speech."
Given that, I think it's generally acceptable to write in ways that may be quite distinct from ordinary conversation. HOWEVER, you never want the way you express yourself to take all the attention. Writing is always aimed at some larger goal, and if people are distracted from that by odd or archaic word choices, that's a bad thing.
If you can use the conjunctive "for" in a way that isn't jarring or confusing, and that serves your writer's goals, then go for it. But I would counsel caution, for it would be a great pity for your writing to be dismissed merely because of your word choices.
