The evidence is now in: the explicit teaching of grammar rules leads to better learning. At least that is what this article from The Guardian in England claims.
Unfortunately the article does not offer any proof that the evidence is in, but rather just makes the statement. In fact, there are other studies, such as this article by Benkiko Mason in Japan, entitled “Impressive gains on the TOEIC after one year of comprehensible input, with no output or grammar study”, which tend to prove the opposite.
Personally, when it comes to language learning, I am somewhat in the middle. I think that massive comprehensible input is a necessary precondition for language improvement, even in your own language. If you don’t read a lot, you will not write well. In my experience, if you read a lot, and listen a lot, in a foreign language, you will improve faster than by trying to nail down grammar rules.
However, I find that output, speaking and writing, that is based on a solid base of input acquired language experience, which includes the acquisition of a rich vocabulary, helps you to notice your mistakes. The occasional review of grammar rules helps to notice patterns in the language, but only if you have acquired some experience in the language via input.
However, a grammar centred form of instruction, with limited time for input based activities, will, I think, be counterproductive.