The issue is not about giving a couple of decoders the same address or creating a consist... ...it's about some of us reaching the limit on our controller in relation to the number of chips the system can support.
By not needing decoders in unmotorised units, you can shunt them in the fiddleyard; and be able to add only a single decoder onto the controller.
For me, if I were to buy three motorised units, then I would exceed the capacity of my control system. Therefore, i would need to buy a new control system, which is a huge unwanted cost.
Taking a system design approach, I'm not looking for a cheaper four car 321, but a solution that I can run/shunt as a 12 car set with one decoder! 😀
Note: I'm also wanting to ensure that @Revolution Ben can stay in business and provide us with more great models! Hence supportive of @ruggedpeak's comments about designing in the lack of motorised unit at the development stage.