Why do tanks need Determination stat still when they have Tenacity?

[Megguido]

Now I'm curious. Time permitting, would you please explain the reasoning behind the above statement and what exactly is meant by "personal damage mitigation"?

Not trying to sound stupid; I'm just trying to see if we're thinking of the same thing.

I mean, PLD self-mitigation is less good than DRK and WAR, but it's the best at mitigating damage on others. Cover, Intervention, Passage of Arms and Divine Veil are mitigation, but for the team. DRK and WAR can't protect an ally or the team so well.