It is completely useless, since the plastic for 3D printers is too brittle. It's as risky as a homemade weapon, both are equally likely to blown up in your face.
3D printer weapons have been tried before, and i saw on youtube an automatic rifle break apart after firing a couple shots, with the chamber broken.
Also, the plastic barrel is likely to be unrifled. An unrifled barrel has a extremely bad precision, meaning that is a crapshoot at more than 10 meters.
Oh, and for those who are worried about smuggling these by the airport control. Sure being all made of plastic means can pass by the metal detectors.
But THE AMMO is metallic. So at most they could smuggle the empty weapon. A terrorist could try to bluff his way, but very little else.