With this type of application, it is very possible that 3D could have real impact on commerce and industry far beyond where anyone would have imagined. Its application is evolving and this sector is just young.
Yes you are right. 3-D printing is no longer just for prototypes, but can also be used for limited-run end-user products. For long runs its still cheaper to use stamp dies for sheet metal, injection molds for plastic parts, and CNC (computer numerical control) machine tools for metal parts, but I predict that the day is in sight when even mass produced items will be using cheap, super-fast versions of todays 3-D printers.
Printing is a great way of producing 2-D objects, but not necessarily a quick way of making 3-D ones.
However, if you're making something from tough, notoriously refractory materials, (which is pretty much par for the course when building rocket motors), it makes a lot more sense to build them up than to turn a large lump of the stuff into (mostly) scrap.