At the prices medical-grade titanium goes for, it is most certainly not wasted. The machined Ti is reclaimed (or at least it would be if I were in charge). Stating that there is 80% waste is marketing hyperbole. A fairer comparison would count the unsintered powder in the 3D build machine, and would end up being unfavorable to the 3D process.
But if you're in the business of making replacement body parts, you might well be starting with a generic titanium casting (or one of a series of different sizes) and machining it down to fit. Artificial hip joints are sometimes made that way.
Don't get me wrong, 3D printing makes a lot of sense for highly-custom items... although one needs to worry about the potential infection and reaction issues given the inherent porosity of sintered material that give purchase for pathogens, and lots of surface area for irritants that will slowly leech out.