When would you take DRK over PLD or WAR?

[Daega]

Not much really. DRK has no utility. WAR can burst damage and heal. WAR takes hits roughly the same as DRK but with the Defiance hp boost it is sturdier. DRK has TP/MP issues as well. PLD is the king of tanks due to defense/cooldowns/utility like cover, sheltron, and clemency? the heal, and still can do decent dps.
Right now there's not and high end game content until Alex and then it'll become apparent how much DRK needs love.

This is coming from a 60 DRK who has seen all content to now. When you're in a grp with a PLD or WAR you can see the difference.

[Rbstr]

Reprisal stacking with Storm's Path is very strong. Parry procs often enough that it's up very nearly on cooldown just about every time in my experience. Used with Dance you can pretty much force a proc when you know you'll want it. That has the potential to really help with low-HP dps in progression.
Thinking to 50 stuff, since that's all the "raid" content we have to look at (where we remember that 50+ skills don't apply). Before nerfs/echo:
Fights with magic tankbusters, obviously. T12's revelations would be very easy.
DRK/WAR would have made a pretty good pair in t13. DRK would swap in as MT where they would do the exact same rampart/sentinel rotation as PLD on flattens. Plus Reprisal. Then Akh Morns are super easy with buffed Dark Mind.
In turn 4 the AOE damage might be nice over PLD.
That's really the case with any level-50 content. The "reliable" CDs match up with Paladin's plus they get the extra magic CD. Pretty easily plug and play for whatever.

I wouldn't care for them in T5, where chain-stun PLD is great and WAR MT is the best choice anyway. Leviathan EX's chain stun comes to mind as well.

WAR takes hits roughly the same as DRK but with the Defiance hp boost it is sturdier.

This is just wrong. If you plop them into the same gear DRK has a minor defensive advantage (2-5%ish) from Grit compared to Defiance. War gets varying parry buff they burn all the time?
It's funny how people used to claim Shield Oath was significantly better than Defiance, but now where Shield Oath is renamed to Grit, it has become the opposite.

[Tila]

snip

You bringing up flare kinda solidifies my point. If you dont have a reprisal up and ready to use, your advantage goes right out the window. And many bosses dont give you much room to get, hold on to, and use reprisal for the big hits. It doesnt mean much if you use it and mitigate a bunch of smaller AoE attacks. What matters if having it for those clutch moments where everyone needs healing fast because a big move hit.

[Hundred]

Dark MT/PLD OT, Ninja DPS.
Magical tank busters or aoe to the party
Drop reprisal, Throw int down, Ninja increases DPS, PLD goes sword oath and hits Divine Veil as needed for the extra prot, Int down and Str down covered.

Dark MT/ WAR OT, throw in Ninja/Dragoon.
War provides slashing buff and 10%, Drk provides another 10% down from reprisal with just under 66% uptime. Boss is doing 20% or 19% less damage, to everyone about 66% of the time, not including Int down.

Add phases that need to be held. Dark handles adds quite well between Dark dance ,Abyssal drain (when not bugged), Dark passenger (when not bugged), Unleash, Salted Earth and Sole survivor