depends on the circuit to be biased and the fact that PTAT (current related to Vt (=kT/q)) is easy to generate.

for example high speed Source-Coupled logic dividers would benefit from more current at high temperature to maintain divider speed/operation so PTAT bias may be better here than zero TC current.

so it is "horses for courses" ... the designer using the bias characteristic that suits their circuit, be it PTAT, CTAT or somewhere inbetween.

Well again it depends on the OPA and the configuration under consideration.
Quantities like gm have tmeprature characteristics which can be compensated for by appropriate bias.
So in the design of an opamp, one may well start with zeroTC bias and then sweep over temperature to see if the varitaion of the relevant opamp performance parameters are acceptable before considering improvements thorugh bias charactersitic tweaking.

yes zeroTC biasing is a combination of PTAT and CTAT quanties. if you comibine in different ratios you can make any temperture coeffient inbetween as well, not just zeroTC (although zeroTC is usually the target)

For good treatment of this subject have alook at Grey & Meyer "analysis and design of analog integrated circuits" or similar.