I've fixed our commercial (China quality) lasercutter using parts made on a cheap FDM 3d-printer. Fixed powertools like drills and drill-presses. Replaced parts on my bicycle. Made functional scissors, and a haircomb that I needed when no shop was open.
The issues are that for commodity items, in the first world, it is quicker to just buy them (if we don't have them already). They will also be prettier. Or if it is a custom item, one has to actually design&test, which is something that requires CAD skills and some hours of work. Teaching this will take some time.
Most people just watch kitten-videos and play Farmville using their Internet-enabled devices. Does not mean the devices are low-quality and cannot be used for useful things.
This way you can combine rapid iteration with high-quality production. When designing new parts, I often do up to 5 iterations a day. Sometimes engaging up 3x 500 USD printers at the same time, to keep iteration time down.
Of course, it's a matter of time before tech advancement fixes this problem, but today there's little reason to buy one of these machines if all you are planning to do is actually print useful stuff, rather than experiment with 3D printers as a hobby.