I think you're fundamentally misunderstanding the point of descriptivism. A language and its grammar don't have to be completely unified to be described, rather you describe them as a disparate set of standards. Descriptivism is a scientific position, not a correctness position.
It's amusing that you speak of standards, then deny that you're concerned about correctness. Obviously, identifying a standard in a certain milieu with "scientific" dispassion still means you are now ready to advise people on the "correctness" of their speech for that milieu. Maybe you finesse the word from being "correct" to "consistent with standard" but you are in essence prescribing usage based on an ideal for that context.
So then, the prescriptivist is for the most part just like you, since everyone intuitively grasps that standards must depend on social setting and readily changes his standards where appropriate; but the prescriptivist elevates, in appropriate contexts, the grammar from the most literary era of English, the late 18th to early 20th century, as somewhat more refined and desirable in formal settings. The 18th and early 19th century saw a burgeoning of reading and writing among the bourgeois, and the literature consumed by the masses at that time became a widely-adapted standard. Most people continue to agree that Dickens and Hardy and Austen are more eloquent that the chap down the road. Thus, your assumption that all standards are equally valid is false, because you can
describe (to use your own favorite word) that most English-speakers continue to respect the English Classics with a certain extra regard.
Regardless if you think common usage is mythical, language is used and that usage is hardly as influenced by prescriptions as some people would like you to believe. If linguistics is going to be a legitimate form of study, it must study language as it is, not in some ideal form.
If you're claiming that the ideal form of prescribed grammar is some unattainable "platonic form" of perfection that never existed in reality, then that simply means your reading list has been a little too light. The prescriptivist version of English existed as a literary reality for well over a century, a century during which the middle class was first introduced to literature on a large scale; and the grammar prescriptions are merely
descriptions (your word again) of the general patterns of English usage found in that respected body of literature.
edit: And yes, I do believe that if the message was understood it was a successful speech act, but that is not the position of most descriptivists. It's an extremely radical position, I assure you.
Me agrees at you.