I just remembered a good reason for this rule on disclosure - boundaries! A therapist has many clients and has to listen to some sad stories. In order to do this there has to be good self care. Boundaries are part of that self care. Also, one of the best tools that a therapist has is himself and how he interacts with the clients. He has to put himself out there to interact and connect. The therapist just has to stay "packaged" you know.