Teams need 2 goaltenders on the roster. If the backup is a useless league min plug then you need to ride a starter heavily to balance out the weakness, making them of greater importance. No different than if the third and fourth line give no scoring, you need your first liner to really carry a heavier scoring load. If the backup is good, you can have a lesser "starting" goalie because they can split a more even workload.
Essentially, all 64 "at a given moment in time" goaltenders need to be evaluated to understand a particular one's place, same as evaluating 384 "at a given moment in time" Forwards.