Combat AI: use "WhenUsed" enchantments
Current implementation of combat AI does not use WhenUsed enchantments (rings, amulets, "devil" items, etc). It uses only CastOnce (scrolls) or WhenStrikes enchantments.
Morrowind AI uses also WhenUsed enchantments with hostile effects.
Note: it seems Morrowind AI can use WhenUsed item with the same ID only once per combat (maybe to prevent spells spamming - enchantments have no animation).
I do not know if we should replicate this case. Maybe just take in account charge level when rate WhenUsed magic items?
If you want to test this behaviour, you can add to NPC an magic item such as "ring of fireball".