I've used Holmgang well on Ultima HM. Right at the end, other tank dies and I have still 3 stacks...get a fourth...and then a fifth (he's still dead; everyone is just DPSing). Holmganged just before the fifth and survive those 15K bursts for 6 seconds, then Ultima starts to turn to the DPS, but dies just after I do.

Then there's the times against Behemoth, or Titan, or any other boss, where you just aren't getting heals (dead heals or derpy DPS or something) and need it to survive.

And if anyone still runs WP...use it on the slime boss to hold it in place and save the poor healer from Flail.