> they just don't do anything useful.

We use a cheap 3D printer weekly to make little fixtures, brackets and mounts for robots. These we use for months or years. They are also incredibly great for prototyping: print the doohickey in crappy plastic, check it fits together right, then get it made for real. It's amazing how helpful it is to have a model in your hand for debugging before spending a fortune.

I would amend that to "commercial grade 3D printers are terrible and awful". There are some monsters out there that can pump out amazing stuff quickly, but they come with a hefty price tag. And some use lasers, so that's super cool.

Consumer 3d printing is invaluable for small batch rapid prototyping. I can design and print multiple revisions of a prototype in as many hours. That type of efficiency was previously out of reach(expensive) for tiny, bootstrapped product design teams. It's a huge win.

It is definitely not ready for my Nana to print out a new frame for her eyeglasses after she accidentally sits on them. But it will be one day.

Once you start treating the output as something you can make a mold out of, it makes a lot more sense. Print something out, create a mold, then cast your real parts with that mold in short order.