“Language acquisition does not require extensive use of conscious grammatical rules, and does not require tedious drill.” Stephen Krashen
Grammar rules are a temporary substitute for habits which are formed over a long period of time. They can act as a useful resource for a beginner, but learners will always reach a point when they find exceptions to grammar rules and realize that grammar is only a temporary crutch, which has to be replaced with a more intuitive feeling for the language.
I would argue that the grammar approach to learning is not as useful as people think. A language cannot be reduced to a simple set of grammar rules. Language is more about the development of lexical accuracy through trial and error, recognizing cultural context, and responding appropriately. It is not necessary to understand every word, or complex grammar, in order to comprehend the overall content of what is being said. Likewise, you do not need to replicate a perfect sentence in order to be understood by your counterpart. Grammar rules are best learned unconsciously and instinctively and used intuitively. The role of the teacher is to teach grammar without the students realising it.