Michael Hughes has made an interesting point. "Having been in the teaching of English game for nearly three decades and having used and seen a number of methodologies, I can't really say that any of the methods I used actually failed to teach English to my students. One could say certain methodologies are more boring (repetitive), enjoyable or useful in certain circumstances, but by and large they all achieved their broad aim."
Let's all keep in mind grammar teaching's constant descent from being the all-in-all of English teaching to reaching the point that we are now debating if it is even necessary. Let's remember the old English teaching books in which grammar was central. As Jack Richard's puts it:
"In the 1970s we were just nearing the end of a period during which grammar had a controlling influence on language teaching."
As Michael Hughes points out, the books worked, students learned and "by and large they all achieved their broad aim"...or did they? Certainly many students learned that way. Some students simply love grammar.
But how many failed? How many determined they were too stupid to learn a language because they couldn't remember all those rules and how to put them together to create coherent language?
In 1970, I was one of the stupid ones, too stupid to learn French in school. At least, that is what I decided, the way it looked to me. Or was I too stupid? If a more communicative approach was taken and I was presented with fascinating reading material, audio and video material that was just nearly within my language range (ie: Krashen's i+1), would I have been able to learn French? Consequently, I had to wait about ten years until I was living and working in France before I picked up the language on the street without a book. By that time I had already picked up Spanish in Puerto Rico and Spain in the same way.
Was my French and Spanish good? No, but I could communicate. As Krashen suggests, this would be a good time for some remedial training in the form of grammar training. Thus, grammar teaching plays, at most, a supporting role rather than a starring role.
 http://www.sdkrashen.com/articles/pac5/all.html http://www.sdkrashen.com/articles/singapore/singapore.pdf