This topic was formulated by Win Stone and Don Walsh, and it shows it. There are at least three questions buried—not very deeply—inside it, and there are booby-traps buried inside each of those three questions.
First : Why should a foreign-language teacher teach grammar?
Second: If he should teach grammar, why English grammar?
Third: If he has to teach English grammar, why didn't his students' English teacher teach them English grammar? These three questions are interlocked, and we may admire the Stone-Walsh rhetoric which has homogenized them through a skilful use of noun-modifiers and verb-phrase structure. Nevertheless, let us at least begin by examining the three questions in order, and with some degree of separateness.