It’s basically what it is. Deathmark’s Shadow set skill. It’s hard sometimes to see debuff on enemies and one cannot be sure it’s applied or the casting interrupted with CC especially againts multiple enemies in a messed up combat.
So basically a 3 sec cooldown provides some certainty along with it make the skill bindable for some devotion skills. Which is also very important. AA Cold damage builds has a lot to bind and mostly not enough skill to properly proc things. Most of them uses Elemental Storm and Blizzard, Rumor is mandatory, some uses Bat and some uses Whirlpool. And since AA+ ABB consumes most of the combat rotation (and one of the skill can be bindable to Blade Spirits); Reaper’s Mark skill with CD can provide some flexibilty. Some use Ring of Steel to do that but with 2,5-3 sec Reaper’s Mark, those spent points can bu usable in somewhere else.