Honestly, if someone really, really wanted to make a single-shot gun instead of just buying one for whatever reason (e.g. restricted person), it would be much easier (and probably cheaper) to just machine one out of steel. Some people may argue that they're less detectable, but from what I've seen, the thickness of the plastic (especially around the barrel) means they're far less concealable than a compact or sub-compact, so they're really only less detectable if you're talking about metal detectors. To that end, how many places do you frequent that have metal detectors and no other security precautions, anyway? Airports X-ray luggage and have body scanners, so the lack of metal wouldn't really help.
No, personally, I think 3D-printed guns are simply a novelty. The only new danger they introduce is the possibility of someone constructing one incorrectly or using the wrong plastic and having bits of it blow up that shouldn't. And if we're going to worry about dangers just because of a lack of standards, we should worry about any DIY project.