> The dominant perspective on writing in academia teaches people to do exactly what you are saying.

Dominant perspective among whom? Surely not the entirety of Academia. Very few academic writing I've seen are written in plain (let alone "as you speak") English.

Complex sentences, peppered generously with subordinate clauses and participle phrases, are common throughout STEM, social science and the humanities alike. Passive voice is likewise used throughout. Obscure vocabulary ("The mercurial Spaniard") is perhaps considerably more common within humanities.

Yes, many (perhaps most?) English Writing instructors are advocating a certain form of plain English - one which is typified by your quote. This school of thought dates at least as far back as The Elements of Style. And you'll find most of the admonitions the Hemingway Editor throws at you in there: Write short sentences, shun the passive voice, purge your adjectives and adverbs.

And despite this particular school of prescriptive linguistics (let's call it the Strunk and White school) being dominant in English Writing teaching in the US[1] for over half a century, little came out of it.

It's rather ironic that at the very same time that the Strunk and White school gained foothold with English teachers and the public, English departments in American universities have embraced post-structuralism to such a degree that the very concept of "English departments" became synonymous with "postmodernists" for many people. This post-structuralism is the driving force behind most of the entrants of the Bad Writing Contest:
http://webcache.googleusercontent.com/search?q=cache:www.den...

Now, even if I were to agree that the Strunk and White school I described above is really dominant in practice (and not just in theory), I don't think that the plain English they advocate for is "spoken language". GP has claimed that spoken and written language are two, slightly different, dialects. I would venture further and say they are vastly different.

First, the medium: Spoken language is carried over multiple channels: the phonemes themselves, tone, loudness and rhythm[2]. Out of the channels above, writing only replicates the phonemes (with letters) and an extremely limited approximation of rhythm (with punctuation). Of course, written language has its own unique properties such as division into chapters with headings and paragraphs and styled text: italic, bold, underlined and so on.

The different medium creates a different dialects in practice. Taking your quote from above, I think a more faithful rendition of something that naturally appears in speech might be:

"So, the main advantages of plain English are: yeah, obviously it's faster to write and read; and you also get your message across a lot more, it's a lot easier, friendlier, you know."

I'm not a native speaker, so my rendition is probably not as natural as it could be. And the punctuation doesn't even begin to do any justice to all the missing prosodic elements. But this how spoken language really is.

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[1] I haven't delved into this deeply myself, but I'd dare to guess that the UK English Writing instruction landscape was originally more influenced by Fowler's Modern English Usage than by Strunk and White.

[2] All the non-phonemic elements of speech are often collectively referred to as "Prosody" or sometimes Suprasegmentals.