The following statements are prescriptive rules of English:
i. Its me is ungrammatical; its I is the correct way to express this idea. ii. Between you and me is correct; between you and I is ungrammatical. iii. Never end a sentence with a preposition.
Now, observe the equivalent descriptive rule for each of the prescriptive above: i. Many people say its me although the prescriptive standard is to say its I. ii. Many speakers of English say between you and I, as a hypercorrected form for between you and me. iii. In casual styles of speaking, English speakers frequently end sentences with prepositions; ending sentences with prepositions is avoided in formal styles.
Considering what you know about descriptivism, prescriptivism, linguistic competence, linguistic performance and mental grammar, answer the following question: Why do you think linguists are more concerned with descriptive grammars than with prescriptive grammars?