Originally Posted by wikipedia

However, description and prescription can appear to be in conflict when stronger statements are made on either side. When an extreme prescriptivist wishes to condemn a very commonly used language phenomenon as solecism or barbarism or simply as vulgar, the evidence of description may testify to the acceptability of the form. This would be the case if someone wished to argue that ain't should not even be used in colloquial spoken English. Prescriptive statements will sometimes be heard which suggest that a word is inherently ugly; a descriptive approach will deny the meaningfulness of this judgment. In such instances of controversy, most linguists fall heavily on the descriptive side of the argument, accepting forms as correct or acceptable when they achieve general currency.