but wouldn't he be the poster child darling of descriptivists? He personifies that make-it-up-as-you-go free-wheeling linguism

This was too much for a soi-disant descriptivist, such as moi, to let go by without an observation. The prescriptivists wouldn't know a real grammatical rule if it walked up and bit them on their tukhes. A descriptivist determines what the rules governing language are by describing how people use said language. Prescriptivists make up rules, which more often than not they themselves don't follow (link, link, and link), or borrow them from some other grammar hooligans. The latter use faulty logic, ignore history, and just get the facts plain wrong.

Ceci n'est pas un seing.