Are 4 mid-voltage dividers really necessary?

[arnolde]

I notice each CT input has it's own mid-voltage divider circuit (2x470k). Why can't one mid-voltage be used for all 4 CT's? Also: Why use 2.5v reference at all, why not just measure the positive AC halfwaves between ADC and GND, and ignore the negative ones (or snub them with a diode to protect the ADC input)?