I had a supercab once, did not convert it, but i really don't see what the fuss is about from a safety perspective. You can't even open the doors and there's no openable windows in that space.
You can convert that space with seatbelts and all and it will be safer than loading people on the back of the bakkie (which is something i have a much bigger issue with, parents loading their kids on the back and such).
If i look at my current car, and the back seats of that compared to a supercab that is properly converted i would be very surprised if it's less safe.
That said, i understand from an INSURANCE and a TAX perspective it's a totally different story. Insurance and when you're in an accident there's quite a good chance you will be in very big trouble (whether it saved lives or not) , especially since the conversion is most likely not accredited by anyone and tested etc. I know in the past people did it to dodge some tax , so they got a supercab/clubcab and added it as a business vehicle , not sure if this applies still.
But from a practical point of view, i dunno hey, why does the OP care? It's like complaining about people putting in a sunroof or mag wheels. Chances are if said person wanted to transport 3 people, and that's the car he has, he is going to be screwed either way, no matter where he tries to put the 3rd person in, but at least converted seat is in my opinion the best solution other than using a different car...