to my way of thinking, a 3d printer is just another tool (a very versatile tool, but not the be all and end all of tools), so when i have something that cant be scratch built easily, then the 3d printer comes into use, and its generally for fittings and fixings that i use it, some I will take the time and effort needed to make them in brass on the lathe or mill, but others can more easily be made on the 3d printer, I actually enjoy both processes, as since I am a draughtsman, I get similar satisfaction in producing a 3d drawing of the part required for the printer that I do in machining the part on the lathe and mill, at the end of the day sometimes it comes down to whether I want the part in brass or plastic.