The OED was conceived as a dictionary ‘on historical principles’. The practice employed from the beginning was to illustrate the several sense definitions of a word over time with quotations from published material across the lifespan of that sense.
Currently I’m engaged in trying to quantify the extent of poetical sense illustration in the OED. But clearly it’s significant: the top 5 individual authors cited (as opposed to periodicals – The Times of London is the #1 source overall) are Shakespeare (1, 2 overall), Scott (2, 3), Chaucer (3, 6), Milton (4, 7), and Dryden (5, 12).
I can think of lots of reasons why literary writing might make up a large percentage of the illustrative quotations. For one thing, as OED general editor John Simpson says (here), illustrative quotations ‘often reveal more about the reading programme for the Dictionary and the selection criteria of the editors than about the lexical significance of the source.’ Fine. But by including, say, a poetical source as an illustrative quotation the dictionary editors themselves create lexical significance for that source.
I think what Simpson means by ‘lexical significance is’, among other things, the ability of a usage to encapsulate clearly, succinctly, and unambiguously a word sense as it has been restricted by the definition for that sense. But by most accounts, poetic language tends away from the denotative, literal, and restrictive, and towards the connotative, metaphorical, and expansive. So, given the presence of literary language, and especially of poetic language, in the lexicographical document of record, and given the influence of poetic writing on its editors, on the entries they created and the definitions they composed, what does this tell us about poetry’s effect on the language? Is a usage by Shakespeare of the same lexical significance, or rather of the same lexical status, as a usage in the Times? I don’t think so, but I also don’t think it’s of inferior status. ‘Apple, n.’ is illustrated by both Milton (‘Him by fraud I have seduc’d From his Creator‥with an Apple.’) and Weight Watcher’s Magazine (‘This handy apple corer/segmenter‥will core and slice an apple into 12 even segments easily and simply.’). Which is more significant to you?
And this doesn’t begin to address the question of poetic neologism.