Update tax.php

Just curious, why to check if tax_rate_id already exist in tax_rate_data because against each  call to getRates() method, tax_rates will be traversed only once for the parameter tax_class_id. There is no way that tax_rate_data will already have a value against any Tax_rate_id. Hope I am able to tell my point.
Just curious, why to check if tax_rate_id already exist in tax_rate_data because against each call to getRates() method, tax_rates will be traversed only once for the parameter tax_class_id. There is no way that tax_rate_data will already have a value against any Tax_rate_id. 
Hope I am able to tell my point.