But sometimes, e.g. when I've got a loaded shocking staff and all I need to do is touch you with it in order to deliver its shock, your armour and some other AC factors might not help you. In these cases, I'm rolling against a lower AC.
Do you play 5e at all?nIn 5e the shock spell rolls a regular attack vs AC, with advantage if the target has metal armour.
The default assumption in 5e is that armour protects against damage, and AC is a measure of protection. Where that is clearly inapplicable the game uses opposed checks, although often monsters roll vs AC for stuff like grabbing and pushing as part of their attacks.