Descriptivist linguists tend to do what the label on their tin says: they describe the language they see and hear around them. They don't make value judgements about correct or incorrect English, but talk about what's grammatical and ungrammatical, what's standard and non-standard. But from the abuse they sometimes get you'd have thought they were ramming dodgy grammar and youth slang down our throats every day and forcing us to speak non-standard.
So, spare a thought for Professor Maurice Martinez at UNCW who endeavoured to teach his students a thing or two about what he called "Black English". We all know that the people who post comments on newspaper websites don't tend to be a representative cross-section of society (unless that society genuinely is full of rabies-ridden reactionary mouth-frothers), but some of the comments about Martinez's lessons are an interesting insight into how the minds of knee-jerk prescriptivists function. But it's also good to see some of his students getting on the case and responding with some well-argued justifications for the kind of work their professor is doing.
Have a look here if you'd like to see more.