In my opinion an NEC violation to use a lamp holder to supply a receptacle.
I will also point out that most lamp holders have low ratings, under 600 watts and have 16 or 18 AWG leads on them.
Not a good situation if you start drawing 10-15 amps on them.
As far as the 'not installed argument' I don't buy it.
You do have a good point and illustrate a common sense reason not to use the adapters. However, from an NEC point of view, I am curious....
Is there a difference between 'shall be installed only for use as a lampholder' and 'shall only be used as a lampholder'? I say yes, there is. The former is a part of the NEC, the latter is not.
What's the difference between an adaptor allowing 15 amps on a 16 or 18 ga. lamp lead and an extension cord end allowing for 15 amps on a 16 or 18 ga. extension cord? The adapters I have seen are marked as for their limitations. They also have a UL logo on them.
Here is a pic. It's hard to see the print on the top, but it indicates a 550 watt maximum.
Now, while I cringe at the thought of John Q. Public using these, I don't see the NEC prohibiting it any more than I see the NEC prohibiting an 18 ga. extension cord to be plugged into a 15 amp receptacle.
Plus, I use them too. They are handy and if used properly, like a thin extension cord, safe.