ThrashV Correct, they don't modify the minion damage.
3 reasons I take them. First, you can leech off the damage infernal shade is dealing to abomination. It's a ton of recovery at very low cost compared to stacking health regen. Second, infernal shade when converted to physical can apply armour shred. larger area means enemies have more stacks of shred by the time abomination gets to them and decreases the likelyhood of an extremely tanky mob surviving the first hit. Third, in low corruption the aoe dot is actually enough to clear some trash mobs (mostly the extremely numerous ones) which is fantastic, because the things it excels at killing are also the things that cause the minion AI to do the dumbest things.