Defense isn't how PLD reduces physical damage intake. Seriously ... my BLU can get 1200~1600 defense if I really want, it's not that big a deal as it'll still kick your a$$ due to it's base DMG.

PLD's reduce physical damage through a combination of uber shield blocks (Aegis or OChain) and the amazing DT gear that heavy DD can wear. It's there that RUN fails the hardest. Without a percentage based blocking mechanism nor access to the heavy gear, their effective physical defense is that of a THF or non-counter MNK. Basically non-existent.

That's without getting into the sad enmity system that makes the best "tanks" DRK / SAM / WAR's. Actually WAR is probably the games #1 best "tank", provided they got a Bravura. MT lets you maintain 50%PDT/MDT, capped gear haste and a 6-hit while spamming a moderately damaging WS along with a 20~50% retaliation rate. The more the monster hits the WAR the more the WAR hits right back.