This is indeed a good question, Kyi!
If you're learning to play a music instrument, or long distance running; then I agree. I would, however also consider the learning philosopher, John Dewey's slogan: Learning by Doing, which means experiental learning! With training vocabulary and grammar by the book, I learnt some of the foreign language skills, like a foundation for further communication and readings. But only when I really started communicating I felt also a need for the move from beginner to the more experienced level of conversatin skills - learning by talking, by listening and by writing with a purpose, to make sense.
To me, this is a balanced act of the best from both worlds

@susnyrop - 

Hi Sus!

Thanks for your golden words.

Kyi