The receiver for the AR series of rifles does not need to be that strong. It doesn't handle any of the pressure of firing the cartridge; it's really just a mount for the rest of the components. The other components are not considered the actual firearm -- in a legal sense -- so you can just buy those without running into licensing issues (well, maybe local ones). Just add a barrel, a trigger group and other stuff and you're good to go.

Printing something that handles chamber pressures is a lot more challenging.

It could be a good intimidation tool for stick ups. All it takes is one shot so most people will be scared enough to comply as they would when faced with any other gun or weapon.

On the flip side, the barrel doesn't last fore more than one or two shots if you're lucky. You can't reload fast and have a single shot. I can see it being a useful tool for an assassination. But that single shot better count otherwise you're either captured, dead or you lose the target.

Not very.

However, with printed metal (which exists today) it's very possible to print something more reliable, or mods for an existing gun, etc.