Fireteam medallions pay for themselves. They boost all xp gains by 10% and if you play for the 4 hours it lasts then you'd fill your xp bar a couple times at least getting 2 more bright engrams. Then you can deconstruct the loot into dust. And if you play in a fireteam with one like you're supposed to, it doesn't need to be the same guy activating one each time.
I know what they do. The discussion is WHY are we able to buy performance gain and gear gain buffs using real world currency! Micro transactions, according to Bungie are SOLEY cosmetic and have NO impact on the game.