I think a lot of this is being framed about the wellbeing of the social parents and the knowledge that the child has about its genetics.
There is a long history of human rights violations that are carried out against birth mothers - trafficking, slavery, forms of physical and psychological torture. After the Second World War, we had inadequate laws for trying crimes committed against mothers because CEDAW did not exist. Some of those mothers were revictimised due to being viewed as agitators for wanting to know what had happened to their babies.
When similar situations have happened, in Peru, for example, and mothers in prisons have experienced psychological and physical torture connected to birth, breastfeeding and care of a newborn, it has been possible to deal with those crimes under CEDAW because of human rights recognition of the importance of maternity as a social role.
Certain groups of mothers are the ones at greatest risk of human rights violations time and time again - mothers in mental health facilities, in the care of social services, in prison, in detention centres, young mothers, poor mothers, surrogate mothers, mothers in trafficking situations, mothers whose children are to be adopted.
It matters very much that maternity and motherhood mean something. It isn't about who is seen as the nurturing parent. It is about recognising the long global history of violating the rights of birth mothers in ways possible only because they are birth mothers. Surrogacy and adoption can be wonderful, but they can also be part of a global system of trafficking, slavery and the violation of human rights.
A high profile celebrity couple where everything is done by the book doesn't change the situation for many other women whose maternity is a target for human rights abuses. And this is not a problem existing outside our society - abuse has happened in our prisons, our mental health facilities, and children of trafficking end up in our country.
Information about birth mothers is important in the prevention of trafficking, abuse and slavery. Birth certificates are an international legal document that the UN maintains are needed to prevent trafficking (and to provide nationality). This is far more important than issues of changing social roles in parenting, which can be promoted by society in plenty of other ways.