So if we're going to pick this to death, threaded half or even full couplings are not designed, manufactured or tested for joining connectors. Their purpose is solely for joining threaded conduits with tapered threads.
Yes we all do it and it will be done till the end of time, but that does not make it correct. That is why a fitting is made and more importantly tested to connect emt and rigid to flex and cable. In a similar way, threaded condulets are tapered to match threaded conduits, not emt or flex connectors which have no taper. But we still use them incorrectly all the time and will forever.
My point is that sometimes we do things in our trade which are not correct to the letter of the code or the UL listing, but so what? It is a big improvement over most of the worlds electrical wiring. How about European motor starters and circuit breakers that are over-rated and do not compare to Nema rated equipment. I could go on and on, but we are already bored! The sleeve provides more protection than no sleeve. The more complicated the code gets, the less likely that any of us know it all or comply.
