I won't question that young families and those who plan to be young families would be more loyal to a company with a policy like this. Is it possible that those who can't or won't have families will be less loyal to company like this if the practice results in increased workload during someone's parental leave?
Drawing from a deeper talent pool; building a more loyal workforce; improving morale. All of which help your existing hours be more effective and the latter two mean staff are more likely to do extra work.