What is your view on descriptive and prescriptive grammar in language teaching?
In the short term, prescriptive grammar is the only option, or everybody can just start saying anything in any way. Interesting is the borderline, when things change. For instance do support in English. It was possible to say: 'Went you to the ball?' That changed apparently around 400 years ago (don't quote me on the exact time it started), and quite rapidly. Why should that be? In German you still say it like that, although in Bayern they can phrase it with do support.
Descriptive grammar is not really just descriptive. It is also predictive. It will predict that certain situations will be ungrammatical. That's what I like about XBar Theory.
Proto Indo-Germanic has apparently been reconstructed, and found to have had 8 cases. Russian has 6. English only admits to 3. Where is that going to end?
What mental grammar really decides when what is grammatical is still unknown, and grammaticality is not the only necessity, as Chomsky's famous 'Green ideas sleeping furiously' shows.