It really depends on who you are.

I learned Indonesian more or less from reading an excellent grammar book about twenty times over years.

And then I went there, and to my shock, I was able to speak it.

The high point was when I had an argument with people in the airport about the amount of airport tax and got them to concede they'd done the exchange rate right, all in Indonesian.

I would not recommend that way of learning language to anyone, but I just love grammar books, and I had a lot of spare time those years.

But I do agree with your point. Heck, I probably would have learned faster with a workbook and exercises.