[bane]

Grammar and vocabulary is a learned social construct with peculiarities associated with the social group the grammar is derived from. What is correct in one social group may be wrong in another. For example, my first sentence is incorrect in some social milieus as it finishes with a preposition.

Demonstrating "correct" grammar merely demonstrates memorization of expected social norms ('communicate as the group does') for the group that one is a member of -- or wishes to be a member of. "Incorrect" grammar is not a global quality as all grammar and vocabulary is local with respect to group membership, geography, language exposure, temporal presence, and other factors. In addition there are certain idioms and informal speech present in specific social groups that may not conform to even that own group's peculiar grammar and lexicon!

Grammar and vocabulary are often a type of shibboleth used to demonstrate group membership or to discern group membership of an individual you are evaluating. Quite often pedantic demonstrations of grammarian prescriptivism is merely attempted to assert their social norm on another person of a different social group membership and are often uninformed as to the derivations of the rules they are attempting to impose. In other words, outside of pedagogic concerns, it's a dominance demonstration typical of primates and should generally be considered as nothing more.