If the coordinator can be the secondary coach also, then he's working with the position that every coordinator should always give the most attention to, ... Plus, most of the (defensive) adjustments that get made during the course of the game are secondary adjustments.
Really it's not the secondary that's being tested, ... it's the pass defense.
If the offensive line or secondary is not what the coordinator is coaching, that's something he's always got his eye on, ... Most of the adjustments that are made in a game are in the secondary.