Prescriptive v. descriptive

A subtle and friendly (but also incomplete because friendly) debate about language rules in the Times:

http://www.nytimes.com/roomfordebate/2012/09/27/which-language-and-grammar-rules-to-flout/?ref=opinion 

My response -- which may soon be buried in a long list of responses:

The distinction between useful and useless prescriptions always hinges on loss or gain in meaning. If the distinctions between similar words are eroded, language richness is lost; if "disinterested" and "uninterested" come to mean the same thing, the language has lost an easy way to state an important idea. "That" as restrictive is important to meaning; in written English "which" works all right as restrictive if there's no comma, as in the Thurber and the Lord's Prayer. Erasure of distinction IS the consequence of ignorance -- not of the rule, but more importantly of the distinction itself. We writers and teachers fight for distinctions not because of a fusspot love of rules but because of a love of the inestimable value of the gift of meaning descending to us from our forebears. We are conservative, not prescriptive. And like good conservatives we accept slow and useful change -- the kind that made a distinction between "continuous" and "continual" and added subtlety to language, thus to the possibilities of communicated meaning.