@ehBeth,
Of course Beth, the needs of the children are more important than anything else. However, divorce is an adversarial legal process. Two lawyers arguing in front of a Judge is a very poor way to decide what is best for children (I don't know if you have experienced the process).
From my experience, the presumption that joint custody is the best way forward. It makes the legal battle less likely. A mother would have to have a good reason to challenge the father's role as equal parent, and a father would have to have a good reason to challenge the mother's role.
This seems best for most children. In real life each situation is different, there will still be court battles (although hopefully fewer). Starting with a presumption of equality seems like the best thing.