The case was made as a dc plug in general. Though 12V does definitely not require "earthing", you are correct about that. With earth in this case it is not the ground perse but some connection schemes (and standards) require either a third connector or open return path for DC which this connector cannot provide.
Apparently they decided to make it a standard in 1994 after years of mismatching equipment all over the world, which is longer than I would have ever guessed. Interesting to see. Apparently UL "standard" does not translate well to actual standard. My experience is that most devices fail on the spring center contact, and do not fit for all cars. Anecdotal but still: even the expensive charger I have doesn't work in one car I've owned (opel) due to the misalignment of the outer contacts and it jumps out too much). It says it's UL2089 compatible though so I guess opel was at fault. There are so many "extension cords" out in the wild which hardly work or go lose after time due to bad design or because they are just a bit too wide. I own several of them and have thrown most away.
The funny thing is the standard describes two sockets with differing size which is not helpful in any way and looks like a legacy of some manufacturers made into the standard.
The article also mentions other bad characteristics about the connector and plug like bad contact causing high resistance which is simple due to the design but might also be present with other designs of course.
I mean, it's nice it became a standard as it was in every car anyway since everybody smoked in them but there are way better solutions which do not have the problems I already mentioned and are way better solution for home appliances. If there is not yet a proper standard, why chose one which is not designed for this usage and most definitely was never designed for what it is used for today with all kinds of side effects (good or bad).
