As doubt is the dialectical partner of faith, so wanting to use is the dialectical partner of not wanting to use. Without doubt there is no reason for faith; without wanting to use, the question of wanting to stop is meaningless. This is the excitement and energy of ambivalence, of the natural polarities that will eventually resolve into a bright gestalt.
When the client is aware of the desire to use and chooses not to he is, as you say, experiencing a new level of maturity. I am particularly pleased to hear a client exclaim (either verbally or otherwise): I would love to use . . . and THEREFORE I will not! This is true dialectical understanding, and makes good common sense as well.
It is gratifying to have these discussions. The Gestalt literature on ethics has been meagre; the literature on the application of Gestalt theory to drug use has been even more disappointing. How neat to realize that this literature does not represent the level of Gestalt thinking on this matter!