Robe of Invulnerability

[Agema]

If you want to dump some +protec on a character for 0 encumbrance, I'd generally turn Stone Boots or a barkskin amulet first as they're much cheaper.

If you've got a very critical low-HP mage - say an A4 Pythium archtheurg you're relying on to cast Fog Warriors - a RoI could be invaluable if you think your opponent is going to hammer the battlefield with Earthquake or Rain of Stones, as the other two likely won't confer enough protec to save it. It might also save the mage in several assassination or remote bombardment situations.

[Illuminated One]

If you are using big fatiguing spells you can take a normal armor, too, since spellcasting encumberance doesn't matter that much then.