Sometimes, just because you can does not mean you should. With great kydex comes great responsibility. Ok so let us take a look at what is going on with this picture. This guy had a custom kydex holster made for his AR Pistol. I have seen some Kydex holsters for ARs and while somewhat unnecessary they seem to have a purpose.
Remember this one?
While silly, I could see the justification for those holsters. The holster on the back is just not a good idea. Sure if you are trying to cosplay as some post apocalyptic character from some comic book then I guess you have achieved your goal. I am more concerned with the manipulation and lack of safety using this holster. How would one draw their pistol from that back holster? I am sure many of you at some time in your lives, most likely in childhood, tried to draw a sword and sheath a sword in a scabbard behind your back. We have seen it in movies and cartoons, but we learn that it is not reasonable to draw a sword from your back. The same would apply here. Only much more dangerously. At the range most if not all range officers follow the 180 degree rule. The muzzle must not go past the 180 degree line of the shooting bay/range. Now imagine trying to draw that AR pistol out of the back holster? How would you do it without flagging everyone behind you? How would you safely re-holster it? If someone came up from behind you, would you be able to retain that AR pistol and prevent them from taking it? This holster does not cover up the trigger guard. Someone can reach out and start pulling the trigger without even having to take the AR pistol out of the holster. The AR pistol looks cumbersome. Trying to get in or out of a vehicle with that thing on would be difficult.