Well, I am obviously more negative about duality than you. I don't think duals make you grow. They keep you balanced as long as they are present. In therapy it is problematic because the patient needs to develop and solve his problems himself. The dual will just give you the illusion that problems have been solved, but they haven't. When you are on your own again the spell is broken and nothing has changed.
The thing is that you are not prepared after being with a dual. When you are with a dual you are not even aware of your type weaknesses that much. Duality is a bliss. It's kindof like paradise.
I know that duality can feel great and all that, so in that way it can have a positive impact.