What is form? It is the end result of something. It is the shape something gets into when all its elements have been put in order.
Form is the vessel of meaning. Apply disorder to a work of meaning, and watch all the meaning leak out.
I would love to point out that language cannot work without form – that the English sentence is a form as demanding as a sonnet in its own way – that it’s as natural to human beings to speak in form as it is for poets to write in form. Perhaps Shakespeare dreamed in blank verse.
I don’t limit form to rhyme and/or meter but there’s a very good start for you. They are both native plants in the English-language soil.
People who defend informal poetry, however, also despise the sentence. They work as hard as possible, often as not, to make sure that you don’t know whether a given word is subject, object, or verb. They soothe your outrage by telling you that now you get to make your own meaning, children!
So every argument for formal poetry can be turned around by the obnoxious into an argument against it. It’s the same in arguments everywhere. One reason I hate debate – it turns into theater so quickly, but it’s theater where the spectators think it’s all real.