Question

why was it not a good idea for giving the censors the power to rank and re-rank citizens into different classes

Answer 1

Assuming you're referring to the period of Ancient Rome, it was generally not a good idea for the censors to rank and re-rank citizens into different classes because it bred animosity between the classes, which led to domestic uprisings and disorder. 

Answer 2

It was generally not a good idea for the censors to rank and re-rank citizens into different classes because it bred animosity between the classes, which led to domestic uprisings and disorder.