when I was 3 years old my mother told me to never put my hand near the flame on the stove. she had to tell me that because at 3 years old I could not read. when I was young and just starting out in my profession, I was told to fill all the screw holes in the cover of a receptacle. so I fill all the screw holes in the cover. I don't have to read this I just do it because I was taught to do that. if I install a receptacle in a garvin cover I have to remove the 2 mounting screws that were supplied with the receptacle and use the 2 screws with the nuts provided with the cover. then I use one of the mounting screws that was supplied with the outlet and cut it down to a reasonable length and insert it into the middle hole using all 3 of the mounting holes. I don't have to read that somewhere I do it because that is what I was told to do. now I am an old fart but I still do things the way I was taught many years ago because it is the right thing to do.
Whether what you learned long ago is right or not depends on whether the person that taught you was right or not.
Now complicate that with the fact the rules for the topic of discussion did change some 20-25 years ago.
Before those rule changes there either was no raised covers with the end mounting holes or they were optional and nobody was purchasing the ones with that option. I never seen the mounting screws and nuts included with the covers for standard duplex units until codes started requiring more then one mounting screw.
Before those code changes if you were going to do quality work - it usually meant you at least use a specification grade receptacle if the center screw was going to be the receptacle support - cheap receptacles often would fall apart if only supported by the center screw.
I think if it hadn't been for expansion of GFCI requirements - which always needed extra screws/nuts to support them from a raised cover this idea wouldn't have ever came up or at least would have been delayed longer.
