Ask HN: What are your views on writing like you speak?

I'm a long time reader of Mr. Graham and other essayists like Mr. Welch of stilldrinking.org, they are writers that I very much like reading, and they also tend to write in a very conversational manner- PG even has an essay titled "Write Like You Talk" [1].

When I reflect on the various technical books I've read over my life, I realize that the ones that I am able to get through most easily and pick up the ideas within with the least amount of friction are those written in a more colloquial manner. In fact, I often find pieces of writing that are devoid of any kind of 'voice' impenetrable, especially if their subject matter in nontrivial.

However, through all my years from childhood through college nearly every single teacher/professor I've had has wagged a finger saying "do not write like you speak" at some point. This seems to be the norm, as my younger sibling just told me about similar experiences.

I fail to see the value in intentionally writing in a way that is completely different from how you speak.

Does anyone here have opposite experiences, do more colloquial styles make things too obscure or hard/annoying to follow?

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[1] http://www.paulgraham.com/talk.html