Earlier quoted context omitted.
We just need to be careful that this unquantifiable, subjective rating doesn't include biases.
Everything including "objective" metrics includes bias. And that's before you take into account people outright gaming metrics (objective or subjective).
Now the objective measure itself might have some sort of bias, but at least the rules are set and you're not getting screwed over based on someone's feelings. You can argue metrics, you can't argue your managers feelings.