Has the parents' neglect of the children made them weaker or stronger?
I would have to say that parental neglect made three of the Walls children stronger in the long run, the exception being the youngest daughter, who does not appear to thrive in adulthood. It is difficult to say to what degree Maureen's position as the baby in the family accounts for this different outcome, too.
The Walls children were forced by parental neglect to rely on their own inner resources, in effect, parenting themselves and often parenting the parents as well. To what degree their environment created strong adult personalities and to what degree genetic traits enabled this outcome is impossible to say. And this book is by no means a model for how to raise a family. The parents, while they had their own reasons for their flaws, were really monsters of selfishness, creating chaos and pain in the lives of their children, who managed to survive into adulthood against terrible odds. The stability and success of three of the children was achieved at great cost, and the kind of childhood they endured more usually results in more pain, adult dysfunction, and an inability to parent a new generation properly.
It is too easy to suggest that a person's upbringing "causes" a child to be a certain way in adulthood. To do so negates the individual's responsibility to decide the life he/she wishes to live. Some people react to adversity by cocooning, while others react by learning to assert and handle themselves well at a young age. I would argue that Jeannette Walls became stronger as a result of the difficult childhood she led.
When many people first read The Glass Castle, they especially remember the swimming pool scene in which Rex tosses Jeannette into water when he knows she doesn't know how to swim. He lets her flounder, and then she reappears above the water, gasping for breath, yet alive. This is one way of teaching a child to swim, though some would say that's it's evil-hearted.
Just as in life, one must look at someone's actions as a whole to really determine his/her intent. Rex Wall certainly had an extreme way of interacting with his children, yet in some scenes of the novel, one can't help but like him as he speaks so honestly with his children. For example, when the family spends the night in desert and Jeannette is afraid, he reminds her that she can look to the stars. He even has her claim her own star as they lay side by side gazing up into the dark night sky together. There's an intimacy between Rex and Jeannette that seems to transcend his darker ways.
To "find oneself" is often more of a matter of discovering what one doesn't like to get to where one can say that they know themselves and what they do want. Though cruel and abusive in many ways, Rex's actions do teach the Walls children what they will and won't put up with in their lives. They decide to leave Rex as soon as they can - a decision that Rose Mary, their mother, can never gather up enough courage to make. In this way, their upbringing - and particularly the parenting style of Rex Walls - causes all of the children to become strong in their own right. That's the silver lining of their challenging upbringing.