I'm not by any means suggesting that schoolteachers start accepting "not no", "ain't no", etc., in pupils' homework or denying that standardized language has its rightful place in the scheme of things. In fact, I think the standardizers have become a little too lax and also trendy in recent years, viz. the debacle of the so-called "orthographical reform" in Germany, where I live (don't get me started).

However (and this is a big however), language as it's really spoken by real people is the real thing and standardized language is an artificial construct. There is the additional problem of people speaking in an "unnatural" way for reasons of fashion, but that's another kettle of fish. Dialect speakers are still looked down upon and standard language is still generally considered to be "superior" in some way. From the point of linguistics, it is not.