I personally think that everyone thinks there's some sort of cool down on them but in reality, there isnt. Unless I'm missing some sort of post/proof that bungie officially released saying that. I feel like, with the exception of when they first came out and you were pretty much guaranteed a drop every 4 to 6 coins, that I've been getting the same drop rate consistenly. Usually 1 out of every 8 to 10 coins gave me a drop. There's been some times during the rsid where I would luckily get a 2 drops from the shade