Oh, right, right, yes; it's one exchange generally speaking (though some specifics vary); the benefit of choosing to go with block vs. armor is layering (one best block + one best armor rating), and block's general benefit of all-or-nothing vs. stress mitigation.
Note that you don't have to trigger a "use" of a defensive item (one that does a block or an attack) until you otherwise see that the attack would hit ya, which is a hidden but palpable benefit.