I've only ever found one use for Medals of Vengeance. I was playing Pangaea (suprise!) and IIRC the opponent at the time was Marverni (or perhaps it was Tien Chi; some fragile human mage-wielding nation anyway). I had found (or rather, taken a province with the site) a site with mages capable of crafting MoV's, crafted five of them.
Enter the Harpy commander. I recruited five of them for this test. Gave them each a red bandana, a Medal of Vengeance, and script Hold, Hold, (if I had been defending, I would have added one more Hold to that), Attack Rear.
KABOOM. Scratch a few evil hostile mages and their equally evil guards. I tried it again (with just three harpies then IIRC), but the opponent had put some archers/slingers/something at front with Fire Fliers, and the harpies did sadly died before attacking, and killing a couple dozen maenads this time
Would work wonders against communions too, I think. Except of course if the opponent is expecting you to do it...