In any language, the dialect which becomes the standard undergoes a process of elaboration of grammar and vocabulary which enables its users to fulfil a full range of functions not available to the speakers of non-standard dialects.
In England this process began in the early 15th century, and the consequent 550 years of elaboration have given a qualitative edge to standard English which other dialects do not possess. Nor is it true (as is often claimed) that speakers of non-standard dialect can simply adapt spontaneously to standard: there are often crucial practical obstacles to this.
It's high time educationists stopped repeating this old fallacy, and took note of those linguists who have called for a more accurate formulation of the issues surrounding dialect differences.
Professor John Honey 13 Greenlands Red Cross Lane Cambridge