It has a greatly reduced proc chance from melee attacks/abilities.
The reason its still good for the Bear tank is their usage of moonfire (spell/ranged attack) triggers the proc much more often, and bears tend to have a fair amount of versatility (which increases the damage of the proc)
Not sure how well DK abilities are at triggering it (Howling Blast and Death Coil should for sure...D&D and Blood boil would be more questionable imo)
But yeah, getting your hands on one wont be easy unless ur farming it with a guild group where none of the casters need/want it