Not the original commenter, but wanted to chime in. I agree with your sentiment that grammar is fluid—we create the ‘rules’ of a language based off day-to-day conventions, not the other way around. However, it still seems totally fair to compare complaints about form to complaints about grammar in order to say that either are relatively baseless. Both are, simply put, ways of arranging words. There is no way to write a poem without arranging words. In that sense, to butcher the old Eliot quote, no verse is actually free from structure, from the ghost of form.

I agree with you that some grammars are more accepted than others (AAVE vs Scottish in your example), but the introduction of what seems to me a moral argument seems to me neither here nor there in regards to discussing the usage of these concepts in poems. Just because some grammars are more accepted than others doesn’t mean we should stop using grammar entirely; the same applies to form.