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

[DRKoftheAzure]

This, but to put Specific numbers, it takes an 1449 TEN stat in order to reduce incoming damage by 5% and increase damage by 5%. For the same amount of DET you get a 6.5% DPS increase without the damage taken reduction. It's about +.045% DPS per TEN, +.059% per DET. It's a fairly small gain, but one that adds up when you have 1500-2000 DET.

This raises the next question; why should tanks sacrifice that 5-11% damage reduction that also increases DPS by 5-11%(Tenacity) for a flat 7-12% DPS increase(Determination)? Me personally; I would go for that stat that gives me more DPS and mitigation over the stat that is just raw damage.