Yes, sometimes people quit doing drugs or otherwise "clean up their life" when they become JWs. However, there are plenty of organizations out there that could provide the same assistance without subjecting them to phobia indoctrination, separation from family and society in general, and enslaving them to work for the interests of a publishing/real-estate company.
I can understand a hesitation to destroy someone's faith if becoming a JW has helped them in some way, but I would never recommend that anyone, no matter how dire their situation, become a JW. There's always a better alternative. In light of this, I would say that the harm they do is in no way justified by the small amount of "good" that comes from the cult.