The NY Times Motherlode has an interesting column here about paternity leave It suggests that encouraging fathers to take greater advantage of it will help make them more equal partners in child rearing, and so help the working mother as well as the children.
I have seen a generational change in father involvement. My own father (a 1950-60’s father) was a distant figure who was not at all involved in directly caring for me or my siblings. My husband and our friends his age were more involved in childcare, but there was never any question about who had the direct responsibility (not the fathers!). But my son and son-in-laws seem to be equal partners with their wives in childcare.
So I wouldn't worry too much about the number of weeks taken off. It's the long haul that counts, and I have great faith in this generation of young fathers. I'm hoping I live long enough to see the results of all of their loving attention and support.