Ive thought about it myself.
Speaking for myself, as I havent asked anyone else, it is a way of making sure none of that nasty caffeine makes its way into your decaf or vice versa.
Even with my doserless rocky I do sometimes run a few grams of the decaf through to get rid of any leaded stuff still left in the system.
It might pay for us decaf drinkers to have a separate grinder but its all extra money and space.
For the time being Rocky will be an only child.