Look, everybody commenting this so far has it all wrong. This has its purpose as a main use helmet..
Stack this with the right gear, you can get a LOT of Damage Taken-, (Physical and Magical Damage Taken- included) and with its Second highest rating in DEF: for a helment, with Hp and VIT, its great for tanking. The only other 'straight tanking' option i see, would be Kaiser. So It depends on if your going for more of a DEF/VIT build, or a DEF/DamageReduction build.
I'm not a pld myself, but I imagine you could always macro in an "oh ***" set, including this piece. For example, you know for sure you're about to take a -gaIII spell, I'd imagine you could macro this as part of such a set.
Also as Bew mentioned, the slow'll mess up your recasts. However, it isn't quite -haste. Slow is actually a separate part of the recast/delay function, which is something like BaseDelay*(1-Haste%)*(1+Slow%).
This and the Valhalla breastplate mainly designed for PLD/WAR. Sure you can use for PLD/NIN full time if you please with other homam and swift belt slow wont be noteable. I personally have it a macro for when shadow timer is down, throw these on till your good to cast.
Granted the 5% slow is not nice, if you are a pimp tank who already had Aegis + Defending ring. . . Dring -10% DMG Valhalla body & head is another -10% DMG Aegis is -75% DMG when blocking, that already counts to a -95% DMG taken plus hauticlare that goes to an overall -102% DMG (when blocking attacks) now im sure SE has a cap on -DMG% but a total of -102% when blocking & -32% w/o bloccking and always -45%MGKDMG these armor peices are probably designed for someone who wont need /recast becaus they wont need shadows.
this is for looks with head been such a great spot to fill with enmity as /war i dont think you could even hold hate using this as main, i would use this for a magic set maybe