Its not about not learning grammar but learning the grammar by example and not as terminology and rules. Children learn their native grammar without knowing what an object is. In school you don't learn the "truth" about your language, only the formal rules extracted from observing what the speakers and writers of the language are doing. You could easily do without that. Of course grammar (as in terminology and precisely formulated rules) is useful, but it is not the best means of access to learning the language.