As you say, they are useful for hobbyists and prototypes. They don't scale well but will in future.
Today it is small/medium business territory but not a large part of megafactories just yet.
Production thoughts:
- speed. 3D printers are not speedy, the tradeoff of speed vs flexibility is clear. Iterate the design with a 3D printer, make the mould and produce with injection moulding.
- material. $$$ for capability. Cheap plastics <$500. ABS+nylon $1000-$2000. Metal is $50k - $1mil depending on what is desired. Workable for some businesses. IE specialist mechanics: https://www.youtube.com/watch?v=4jbn0ah3u9E
- Method affects end result. Resin printing is different to FDM printing etc. FDM printing works in layers that make a plastic end result not food safe. So no printing kitchen bowls/ladles etc But that leaves a lot of possibilities out there!
The hobbyist angle pros:
- not that expensive! Mine cost $300.
- handy. Keyholder hooks, odd angle shelf brackets, fixing/upgrading kids toys, etc etc
- can send model elsewhere to be printed in fancy materials. See Shapeways.com
- can print models from others. See Thingiverse.com
Hobbyist cons:
- time for 3D modelling. Unless you already know how to 3D model then this will take you time to learn. I see it as a "mindfulness" hobby, like knitting/crochet etc too.