From the Bungie news post 9-24-15 "In accordance with forum prophecy, The Three of Coins that Xûr sells are slightly modified: Each of them provides a smaller increase to Exotic Engram drop chance when used in rapid succession When used with more time between kills (for example, on Strike bosses), they remain as effective as before" So if your playing the game as normal then the loot drop buff is still as effective as usual. If you exploiting it by killing bosses repeatedly in quick cuccession then it will become less and less effective