There is when statistical distributions are involved. You cannot get the correct standard deviation from 1d{2.5, 2, 3, 4}--because "2.5" is not actually a value you can roll on the die. It will give you the correct average, but it will not give you the correct SD. SD of 1d{2.5,2,3,4} is very close to 0.829, while the SD of 1d{1,2,2,2,2,2,3,3,3,3,3,4,4,4,4,4} is almost exactly 0.927. The informal simplification works if the only thing you care about is the average, but it doesn't work for everything. Which is why I said what I said.