The decile system was introduced to guide equity funding to schools and in general it has worked very well.
But there are two distinct aspects of deciles that need to be looked at separately.
Firstly, there are clear links between socio-economic status and educational achievement. Schools from areas where parents are less financially resourced do receive greater funding and there is much evidence of the excellence among many of these schools to transform the learning lives of these children.
I have no concerns about the components of how deciles are determined (equivalent household income, parents' occupation, household crowding, parents' educational qualifications, and Income Support payments received by parents). This equity funding seems a basis of a civilised democracy that wants to best help all students in schooling, regardless of the income of their parents.