The children would benefit by automatic joint custody by having access to both their parents, each parent can be involved in their child's upbringing which is surely a moral thing and right. It is safer for the child when open access is normal and any responsible parent would agree with the importance of this. Child care would be more transparent for the child involved.
There would be less state expense with Legal Aid and less stress for those who have to struggle to find the money to pay for battles for access rights to see their children especially when the access is denied for spiteful reasons only!
Consider the high profile cases where children have been harmed by their parent with sole access, the Mother is ,at will, allowed to let any stranger she wishes into the child's life. In many cases Social Services fail a child. Very often, the Mothers latest boyfriend has more access to a child than the child's natural father.