I was going to start a thread on this but found this one. I have four toons up at 70 and doing a lot of stuff each week (rares, Scout packs, disturbed earth, WQ, dungeons, raids, World bosses, etc) and despite having unlocked DoK on at least 3 of the 4 toons I have not seen a single one drop beyond the ones you get for the initial quest.
Being rare is fine, but it honestly feels like something is broken at this point. If there is some other hidden mechanic governing this it really needs to be made more transparent.