Are the stems of rehydrated dried Shiitake usable? The stems of the rehydrated Shiitake mushrooms are tough, even after rehydration. 
In the recipes that I checked it was advised to cut and throw the stems and keep the head of the mushroom. 

I want to understand why the stems of Shiitake are not eaten. Are the stems edible? Are they usable in any way?
Is there any actual known (perhaps documented) use of the Shiitake mushroom stem? Like for example a dish in Japanese Cuisine that uses stems of Shiitake Mushrooms.
If they are so tough that they are unable to be cut, or unpleasant to eat, I use them in stocks and soups (they would have to be removed before serving).