if someone wants to learn all the grammar I suppose they can go to teacher who specializes in that field. It is a NOT a pre-requisite to become a good speaker

No, but it is a pre-requisite of taking an exam that gives them a certification in English that in turn helps them to get a good job.

And I have a feeling your idea of an upper-intermediate speaker and mine (as a Cambridge examiner) are two different things.

My knowledge of grammar is enough that I can get a class of adults all attaining discintinction pass marks (90%+) at a CAE level exam, so I am pretty confident that it's a little better than 'pretty good'