At the moment, printing technology is most suitable to produce parts that would be impossible to mold in plastic, too delicate for resin (or someone hasn't thought of doing it yet), and too 3-D for photo-etch.
I can think of a couple:
- US periscope guards for tanks
- Those pie pan shaped thingies that go on the Millenium Falcon's landing gear legs. Vector design will allow the base pattern to be scaled to fit the various kits out there.
- The thrust deflector vanes in Fine Molds' Y-Wing Fighter. (The Bandai kit has them properly represented, I think.)
- Afterburner cans and flame holders for kits that don't have aftermarket yet, like the Monogram F-102. Design the breakdown such that the useable portions from the kit can just slot in without sawing and modding.
- And if articulating printed parts are doable, motorcycle chains!