Well, its not like the coupler flexes very much. But if it doesn't flex, the motor shaft and the Z-Threaded_Shaft have to be ---PERFECTLY--- aligned. And you never can do that. If you put that solid metal coupler in place, when you turn the Z Axis, you will feel spots that are very difficult to turn. And that is because the threaded rod has to bend because the coupler isn't flexing.