It is not only important for us to know what to teach but we also must know what NOT to teach.
Our time with our students is limited. So we should definitely focus on what English will benefit our students the most in the period during and shortly after our training.
I've met teachers who say that someday the student may need a certain bit of obscure English or English skill. The problem is that it is likely that the student will forget it before he has a chance to use it. In that case, all of the teacher's and the student's time and effort is wasted.
This is the case with a friend of mine who came to China to learn Chinese. After three years of intense study at the university followed by three more years of living and working in China, she discovered she had forgotten one year of the words she had learned during her training. Because she was living and working in China and very actively used her Chinese, the only way she could forget them was that she didn't need them. She wasted one year of her life learning what she didn't need.
I think if teachers were paid by what English their students remember and are using one year after their training then many teachers would teach quite differently than they do today.