Where? What fights?
It's only useful for absorbing burst magic damage in one spot.
No, those are the situations it's "performing above expectations" in a way that Blizzard can't really fix without making some extremely clumsy checks on.
Elegon's Total Annihilation isn't a particularly good example of "burst damage with raid stacked" either because TA is not just a joke, but unless you're finished a phase transition people won't be stacked up where you are (unless you're by the ranged).
Remorseless winter on elegon adds, that's it. Also useful in a clutch on Empress adds, depending how you handle them... even then it's not a raid CD, since in both cases it only helps save my life, and doesn't do anything for the raid itself.
Gorefiend's on Feng Heroic adds, that's it. I've seen discussion about Maddening Shout but with Undying Shadows it can instantly cause a wipe.
So really, in simplest terms, each of the DK's talentable CD's are each only useful on 1 fight in particular.
They have use on Elegon, Feng, Will, Shek'zeer, Lei Shi and Tsulong, quite probably Protectors and Sha Heroic and that's not trying to pad with fringe cases.
In a perfect world, absorbing 140k of ticking magic damage (which is possible on most fights) might be worthwhile, but Purgatory is a "somebody messed up but it's okay for now" CD and honestly it's usefulness vastly outweighs AMZ in almost every situation.
That's fine for you to have that opinion. THat is after all the entire point of the talent system - to make choices like that.
But concluding that because you feel it's not as useful is not the same as the raid cooldown not being worthwhile just because your raid leader instinctively never calls out for it (Hell, our old raid leader called out "Barrier" for every situation that demanded a raid cooldown, but that didn't mean Barrier was the only useful raid cooldown, it just meant he was a Priest - just like half the time I say DP up when I'm actually meaning GoAK up).
Not does it translate to Death Knights needing "more" utility.
even then it's not a raid CD, since in both cases it only helps save my life
That logic doesn't actually wash. Using an ability that affects mobs in a way that helps you and by definition your raid doesn't somehow change it into "personal cooldown".
The point of a raid cooldown is that you can use it not just on yourself. There is after all nothing stopping you from RW'ing or GG'ing for control for the raid's benefit (Snared/Stunned Adds don't do X), or on adds that are being tanked by the other tank (especially if your raid's strat on Shek'zeer involves killing all on one tank or split-kill-collapse, or anything like that)