How about glasses (frames)? smartphone cases? there are tons of small accessories that can be printed which can save hundreds or even thousands down the road with a 3d printer.
The problem is that plastic molding is already cheap if you're creating a standardized size. The real issue for 3d printing custom objects is that the design is not free. Yes, once a design is done, great. But if you're going to be making a million of them, then more traditional processes are going to be cheaper.
1) people aren't manufacturing, they are producing for themselves and close friends, so volume isn't a cost factor as much as it would be in a production scenario.
2) printing allows for us to get away from standardized pieces for the sake of homogeneity and production cost. These sorts of manufacturing techniques finally allow for the prioritization of artistry rather than the adherence to standards for the sake of cost, as it's putting that decision into the hands of the consumer rather than the business interest.
3) 'design as a cost' is dropping drastically with the advent of tools that allow us to digitize objects into virtual models near instantaneously.
it reminds me of ordering a car from the factory in the sixties.
Back in those days you could order a car with as few options as you wanted. If you were a racer you could order the economy car with the big engine, no radio or ac or carpet/headliner/backseats and have practically a drag car from the factory. Production cost savings of course nixxed that idea of one-off production cars.
Maybe these kind of technologies can allow for those sort of unique configurations without ruining bottom lines too terribly?