i'm thinking an analogy: poetry language is like the media you're using painting a picture. 'media', i mean as the type of paint you use, and the non-paint materials too. like, you can make a painting using oil paint and watercolor and house-paint paints. visualize how that goes down: you've got some darker, richly colored oil paint words, and some transparent magical watercolor words next to each other. myself, i'm using oil paints and colored vinyl tape -- like mondriaan used -- to make paintings. i'm still calling what i make a 'painting', but some people look at my stuff and say to me "that's not painting".
so, in a poem, i can use university language -- robert lowell language, say -- along with the street talk i use with street friends and the philosophic talk i use talking with philosophers and physicists. this is how poetry's always been done: a mixture of careful narrative and the words people actually use with each other.
in a painting or a poem, i want to suggest or evoke a mind-altering experience. we all do, whether we want to alter someone's abstraction, someone's subjective idea of what's supposed to be -- change their mind about race or face or relationships. or, trick the mind into a state outside opinions and bias. two ways of writing: you can stack words and ideas on each other linearly, or you can associate concepts and make words from that... 'invent words'.
when people say we're 'ungrammatical' in a poem, they mean that there's only one language or media for writing poetry. poetry invents grammar the way visual artists invent what painting is.