Learning Joseki Loses Two Stones Strength - Studying Joseki gains four stones strength.
This often cited proverb is intended to visualise how learning Joseki by 'rote' is useless or even worse. The aim is not to be able to replay a sequence, but to understand what each move does and how this particular sequence affects the whole board.
Hence, studying Joseki does help you improve, because it increases your understanding of the game. However, see joseki/discussion for a lively debate on this.
Fujisawa Hideyuki shows an example of where blindly following joseki is not good.
White 1 - White 7 is joseki. However, White 7 is kikasare. It is too passive. White should make use of her strength on the left and play the boshi at a, as in the following diagram.
White 1 is an example of leaning.