Because, like it or hate it, tank dps totally matters -- even if you are the "defense tank".
What's silly to me is that PLD players are always whining about what essentially constitutes doing their job. Good performance of the tank role REQUIRES (regardless of meta, actually) that a tank do the following:
- Maintain hate,
- Stay alive through proper use of abilities,
- AND do as much damage as it is possible for a tank to do.
The concern right now (although, it's been halfway fixed in 3.1) is that PLD MT damage is lower than WAR MT or DRK MT damage.
Why in the world so many PLD tanks want to complain about combos that do damage is beyond me. It's such a ridiculous thing to complain about...
The main intent of this skill, I imagine, was to give PLD something in raids to mitigate raid-wide damage. In other words, it works best when you coordinate with your healers and use it to help with hard-hitting raid boss AoE attacks that the group generally is stacked up for. If you fail to coordinate with your healers to allow proper use of the ability, that's your problem -- not a problem with the design/intent of the ability.
The CD isn't really that long, considering some of the "best" tank CDs in the game are 180 seconds, whereas Divine Veil is 150 seconds. Over the course of a 8:30 fight, that's 3 uses of Divine Veil. That's the same recast time as Vengeance (WAR skill).