A PLD caring for dmg mitigation would have other sources of it anyway (magian sword, shield def bonus, phalanx, various accessories and armor, not to mention +1/+2 body's 2~10% dmg reduction). The set bonus is exactly that: a nice bonus that goes off every now and then. Hell at the moment, I've only got two pieces of +2 and the set bonus fires off at a decent enough rate for me as is.