I believe I need to make my question clearer.
Let's assume a simple application with load shaft and a hollow cylindrical load. The load is fitted to the load shaft through the cylindrical hole on the load. The motor shaft is directly coupled to the load shaft, such that when the motor shaft spins, the load shaft is also made to spin and the load also spins. So the motor shaft, the load shaft and the load spin at the same speed.
For this application, the inertia that the motor would work against as presented in literature would be: load inertia and coupler inertia.
My question is, does the weight (or mass) of the load shaft not matter? Why is the load shaft not considered (or seems not to be considered)?