I regularly find myself needing to make ultra short run parts i.e. a few every year. Not nearly enough to justify doing them in a CNC mill and I don't want to spend a lot of money on a few parts only to have them sit on the shelf because that's capital that could be better spent elsewhere. One of my real world examples is a custom electronics enclosure. The boards I need to house sadly are just a little too big for a COTS enclosure from Hammond or Bud and the next size up is ginormous. So I decided to look seriously at 3D printing. Sending the design to Shapeways or Protolabs was insanely expensive at over $600 for a brick-sized part. That may be fine for a one-off research project but not for short-run production. Then I looked at the sub-$3k offerings. I was at first impressed with the CubeX until I learned that you don't feed it with rolls of filament but instead have to buy their cartridges and they refused to tell me how much material is in each one. They said "Oh, you can print about a hundred cellphone cases." GAH! A cellphone case is not a standardized unit of measure. So their business model is stupid for the customer. Then I looked at some others that could handle the size part I needed to make and discovered how slow they are. I figured it would take about 8 hours to print one enclosure. Well, I suppose I could click "Print" and go do other things. But then I wondered how reliable the process is and I realized that I'd be pretty pissed if the print screwed up 7.5 hours into it. That possibility is pretty much confirmed by the fact that you can now get a shredding machine that recycles your failures into new filament. Finally, the quality of the results came into question when I read about people buying home fryer machines, filling them with acetone, and dipping the parts in to smooth the surfaces.
I still want one but IMHO, 3D printing is at the same stage computers were in the late 70s. Back then, if you were a geek, you totally wanted one but nobody did much real work on them. What's needed is the IBM-PC or Macintosh of 3D printing.