Well that's pure damage reduction there, and I don't have a Defending Ring laying around and haven't gotten anything better from the Dark Rings. But yeah, I'd opt for an OAT/OAThree Khanda and some Haste in spots over the miniscule amounts of PDT would obviously be better for most situations, to get TP up faster for Sanguie Blade (assuming you have a good set for it).
ACC Capped situations where I'm melee tanking, I'd replace certain pieces with TP pieces: Acuben's Helm, Tiercel Necklace, Swift (Goading/Speed/Velvious if you have it), Dusk Gloves (+1/Ocelot Gloves), Homam Feet, etc.
Anyway, in regards to the OP: PDT is nice if you're fighting mobs as BLU/RDM (hi2u Phalanx and Sanguine Blade boosts, Convert, etc), that won't hit you for ridiculous numbers like 250+, where PDT+Phalanx+Protect+Cocoon will actually nullify most of the damage.
Otherwise if the mob is hitting hard enough where PDT is not viable, you either should not be tanking it as BLU (unless you are actually set to be a real BLU tank), or you should be /NIN.
As you may or may not know, BLU is now the new RDM as far as tanking goes, we are now the best un-engaged endgame tank. That being, obviously, anything you're tanking where you will not be meleeing the mob. May I refer you to this for those who have not seen it by now:
http://www.bluegartr.com/threads/37619-Blue-Mage-Job-Findings?p=3969884&viewfull=1#post3969884