The other week, a friend brought over some pre-grated Parmigiano-Reggiano to cook with. It was from a nice cheese shop, and you could tell that the cheese had been grated very recently. It tasted good and wasn't dry or dusty. I was impressed with the quality, and the convenience factor was pretty high.
But we didn't use all of it. And now I have one more reason why I will never buy cheese pre-grated.