1...

Some sockets do have a little bump inside to engage the holes in the plug.

Some even have a lock that engages it with a mechanical release, I've mostly seen it on extension cords, they lock when inserted and there's a button on the outside to release.

2...

I question this one, but it wouldn't "shock me" if somewhere far enough back this is on a patent or a design document somewhere.

3...

I believe that's how these work...

https://www.amazon.ca/Master-Lock-S2005-110-120V-Lockout/dp/B001HWF4ME

I've never used one though, only the "clam shell" sort that fully encloses the plug.

4...

This one seems the least like. I'm sure if you were making large quantities of plugs, you'd sell the bits for scrap, but I can't see the company making the plugs also being the ones recycling the metal, so I assume you'd only be getting scrap weight price for them.

There's a extra cost associated with punching them out, vs not punching them put too.

I would say to possibly get something like this so that when YOU are not using it you can lock it so someone can't turn it on by ""accident""

http://www.amazon.com/gp/product/B001HWF4ME/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687762&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0014ZXWCS&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0GTP79YBQ8VTABXAD6MZ

Unfortunately he's right. The hole is there to be locked and tagged. You will have a hard time finding any outlet using a spring loaded ball to retain the plug - they will all use simple spring clips. Further, the operation to punch the holes and the quality issues with aligning the holes means that it's more expensive to have the hole than the teeny amount of metal saved. You know a little too much, please humble yourself. Like, how can you possibly bring up the lockout/tagout for industrial machinery that will never, ever use a NEMA 1 plug for single phase? You can even find name-brand industrial lock-out devices for $6 on Amazon that use the lockout holes to lockout the plug.

Source: I used to design plugs and receptacles.

This kind of lock is cheap and easy

Both holes are used in industrial lockout devices to lockout the plug. Here are some on Amazon: One Two. The spec says that the holes are optional but if you do manufacture them then you must locate them as specified. Those plug lockouts I listed will not work right if the plug doesn't have holes in the prongs.

So if I don't want to spend $30 on a hoity-toity Grainger lock-out device, these standard Master Lock industrial lock-outs I can buy on Amazon for $6, which use the lockout holes to lockout the plug, don't exist?

You have to read a "Bible standard" like the document you posted as though you are the manufacturer of the plug, not the lockout. The only thing it's saying is that the hole is optional but if it is added then it must be in that location.

I can show you multitudes of real-life industrial lock-outs using the holes as their means of locking out the plug. These date back for over a hundred years. For example, this patent from Master Lock which you can buy on Amazon here

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So, case closed, the holes are there for lock-out tag-out.