Why "improper"?

Over at Language Log Geoff Pullum writes an interesting post looking at adjective (or possibly flat adverbs) used to modify other adjectives in grammatical Standard English (e.g., "precious few"), and concluding (making an excellent point about the value judgments so often seen in those who aren't content to say things are "non-standard"):

I do not have a full understanding of the situation; but I do have strong grounds for thinking that the laziness-and-ignorance hypothesis is flatly wrong (or flat wrong, if you will permit what I think is probably a non-standard idiom). The notion that !real comfortable can be explained as a consequence of ignorant people failing to distinguish adjectives from adverbs is more than absurdly simplistic; it comes nowhere near representing the facts correctly for either Standard English or the non-standard dialects. It would predict complete random vacillation between adjectives and adverbs in all contexts, and we never find that in any dialect.

At the end, Pullum quotes from Bishop Lowth, the man who wrote the1762 book A Short Introduction to English Grammar, which really introduced personal-judgment prescriptivism into the English grammatical literature, and looks at the kind of value judgment he made:

Lowth admits that “exceeding, for exceedingly, however improper, occurs frequently in the Vulgar Translation of the Bible, and has obtained in common discourse”, and he cites Many men reason exceeding clear and rightly, who know not how to make a syllogism from the work of John Locke (to the very learned Dr Lowth, Locke’s dense philosophical work was “common discourse”).

What was going on here? Lowth was a brilliant man: a professor of poetry at Oxford, a Doctor of Divinity, an ordained minister, a translator of Hebrew poetry, and ultimately Bishop of London. He was looking at clear evidence from published writing by the finest writers in the English language — William Shakespeare the Bard of Avon; the great essayist and critic John Dryden (who invented the prescriptive disapproval of stranded prepositions); the satirist and novelist Jonathan Swift; Joseph Addison, co-founder of The Spectator; and the translators of the King James authorized version of the Bible. The evidence was that adjectives sometimes had adverb uses (or co-occurred with zero-derived adverbs with which they were homophonous). Why was this “improper”? If we knew exactly why Lowth thought there was impropriety here, we would know a lot more about the origins of prescriptive impulse that beats in so many English hearts.