I’ve previously approached this stylistic distinction through a construal-level-theory analysis; linguist John McWhorter implies similar conclusions by his insights into the most informal written forms, such as text messaging, through which he highlights the distinctive character of conversation and from which derive many of the customs of social media. With educated talkers using sentences of only 7 to 10 words, the “grand old defining properties” of spoken language (due to talking being “largely subconscious and rapid,” writing and reading “deliberate and slow”) are “brevity, improvisation, and in-the-moment quality.” As McWhorter (“Talking with Your Fingers,” (April 2012)) assesses the state of contemporary language, “Two forms of language coexist in societies: choppy speech and crafted prose.”
Some combinations of the two forms succeed. McWhorter mentions one example: the anthropological novelists combined formal prose with dialog imitating speech. Another intermediate form is of key interest to legal-brief writers: the practical style adapts the formal-prose style to expressing belief rather than opinion. These are careful exceptions to the general rule that combining informal and formal styles is just bad writing. (The “formal-prose” style shouldn’t be confused with the “writing formalities,” which should be compromised.)
But the “plain-writing” trend advises writers to craft choppy prose! Trying to satisfy simultaneously the formal ideal of far-mode clarity and the informal ideal of near-mode immediacy and spontaneity is usually misguided, and perhaps it is also misguided to combine them successively—in different pieces. Can you be a master of both styles, while using them at different times for different purposes? Maybe, but probably not. Each style has its own habits, and writers who practice a great deal of conversation (whether by talking or texting) often seem to do so to their writing detriment; and the reverse, formal writers may deteriorate as conversationalists.
Improving at one task (such as conversation) conflicts with improving at another (such as formal writing) when they call for similar but different responses to the same or similar situations. An example of tasks calling for different responses to the same situation is typing using Dvorak and QWERTY layouts: if the task is typing a comma, you must type what would be a ‘w’ on a QWERTY keyboard, and you will lose proficiency in making one response by learning the other. An example calling for different responses to similar situations is executing a forehand drive in tennis and table tennis: practicing one harms the other. Learning a task negatively transfers to the other when the latter requires inhibiting the response first learned; the extra effort to inhibit the behavior previously practiced makes it harder. If you practice Dvorak, you’ll have to inhibit the habit of typing ‘w’ when you type a comma; if you practice tennis, you'll have to inhibit your tendency to minimize wrist action when playing table tennis.
McWhorter explains, “Spoken language is fundamental, while written language is an artifice.” The habits, even instincts, ingrained in talk are the primary targets of inhibition in crafting formal prose; practicing talk, whether by actually talking, texting, or writing in the plain-talk style, harms your formal writing. But, just as some few may productively use different typing layouts, individuals probably vary in the harm to their formal writing due to negative transfer from conversation or informal writing.