Hmm... I probably didn't explain this very well. You are correct that memorising phrases is not enough. That gets you a kind of grammar dictionary in your head, where you have a single example of that grammar. It will allow you to recognise the grammar, but not use it fluently.
After that, you need to be exposed to many, many different examples of that grammar so that you can understand the context in which it's used and the variety of shapes that it takes. The easiest way to do that is to read (and when I was "studying" Japanese I would read 2-3 hours a day).
In my experience, you need this exposure anyway, because memorising grammar rules allows you to construct sentences, but it does not allow you to understand how to use it properly in context. You have that awkward situation where you know you've said something correctly, but people are staring at you in confusion anyway -- because nobody says it that way.
I'm certainly not against learning grammar, if you enjoy it, but my experience has been that memorising a single exemplar and exposing yourself to countless examples (that you can understand) will bring you to fluency faster. There are an infinite number of ways to make grammatically correct sentences, but the idiomatically correct sentences is a very small subset of that (and actually disjoint since quite a lot of idiomatically correct language is not grammatically correct). Learning by example allows you to reduce the problem space dramatically.
Having said that, I know a lot of people really enjoy the process of learning languages by using grammar rules. If it works for you, then that's obviously the way to go!
