First, I think Ruth has idealized the role of a father. Not knowing her own, she had a somewhat unrealistic expectation of what a father was and of his role in the family and her life. I think the importance of having a dad is that a young girl has the opportunity to see how a man treats his wife and responsibilities with regard to the home and the children, and to show unconditional love and support- -which in turn provide a strong sense of self and confidence. This could help in the forming of sound judgements in determining who would be a good life partner. However, dads are imperfect beings, often struggling with their own issues. More often than not, dads do let go, and we fall. Ruth idolized her grandfather and treasured the special time they had together, particularly fishing at the Wabash River. It was hard for her to learn in the story that he made "unethical" decisions and acted in a way that disappointed her at his job- -even if he was ultimately doing it so Ruth would have a better life.
I agree with ashleighp. I believe children need to know the truth about an absent parent- -as much as they can handle at any time- -if only to prevent the child from believing that he/she is the reason the parent left.