RAF jets fly with 3D printed parts (bbc.co.uk)

The US Air Force is starting a research program to produce 3D-printed rocket engines to “launch critical assets into Earth orbit.” : http://weirdgrants.com/#g19

Why not? If the manufactured parts meet the same technical requirements as traditionally manufactured parts, go for it. It's practical for non mission-critical parts, but it's not like they're printing replacement parts for engines or major structural components.