That is cool, it is not why the system is here, it is not so that you can farm it and get that item you wanted. It is so, that if you run heroic dungeons while helping a friend, or as part of your gearing process, you might get lucky and get a piece with +20ilvl, maybe more or just +5. It is random and low probability especially because they don't want ppl to farm it.
It is easier, if you're a ok player, to run Mythic+ all week and be sure to get an item with the minimum ilvl of the M+ level you're selected (+ maybe upgrades too).
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Per instance, or overall ?
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They'll get ban eventually