Well the Iaso Mitra itself is only really a Side Grade on WHM, admittedly 1% cure potency & 10 Healing Magic which doesn't make it amazing. However that only goes for WHM, when looking at options for RDM or SCH the Paean/Iaso Mitras are the only Cure Potency head pieces, making them very important for those jobs. I myself play RDM as a melee job as much as a mage, and so far as I know Iaso is actually a requirement to cap Cure Potency without needing to change out your weapons, and sacrifice your TP. Admittedly I know some of the gear is just a bunch of side-grades but really in the end it becomes side-grades for certain jobs, and upgrades for others, however I feel Iaso Mitra is probably the best case of this I know of due to my main knowledge being about mages, or more specifically, RDM.