Teach the language, not about the language
Teach the language, not about the language is an axiom in second language teaching that it is better to focus on teaching students to use the language (e.g. English) communicatively instead of teaching students about the language (i.e. linguistics, or grammar)
As an advanced speaker of a language, the teacher's conception of the language may be linguistic-based. However, the students are (probably) not as advanced as the teacher. Of course, students need to know some grammar, e.g. they probably ought to know what a noun is - but how much results in the question of how much grammar students should know.
We need some.