It's like saying the greatest use case for computers is that you can program your own accounting program to do your own taxes. No. I went to the computer history museum, and they had a model of "easy to use" computer for moms in the kitchen. What was on the controls? Loading and unloading registers. They were completely off the mark on that one.
Chances are, it'll enable makers to offer services that have a real world component that other people can order easily, and enable new kinds of behavior.
But to the originally stated use case of fixing broken parts, I don't think that will happen as kids aren't usually running around fixing parts. And for the rest of us, only some of us have enough umph to learn how to 3D model. The vast majority need something easier to make the fixing broken parts scenario a reality.
However, I guess I can see a scenario where older people hire kids to fix parts, like how older people hired kids to do websites in the 90's.
The top to my blender is made of two materials, a very soft pliable plastic that is food safe and a ridget clear plastic for the handle.
These two very simple examples have at least three very different materials with very different properties.
It is going to be one very expensive printer that could work with lots of different materials. Even if it could be made cheaply, small batches of each material in custom colors and sizes would also be very expensive.