This is already happening. A TV program a few months back about forces in Iraq. The head honcho was talking about efficiency of supplies and stores. He said one of the big differences was they now only keep half the spares they used to for trucks and stuff because they can 3D print most of what they need on site. He didn't elaborate on what exactly they are printing but he did mention saving tens of thousands in transport costs and storage.
I wonder what spares for trucks he is printing. Plastic axles, plastic brake callipers, plastic gearboxes, plastic suspension and leaf springs, plastic armour plating?