I think for the 2020 code I will submit a PI that says you can't have a "standard" coupling between an enclosure that is required to be explosion proof and its seal fitting. I don't see as much issue with standard conduit couplings in other parts of the conduit run.
I've been giving this a lot of thought. I still believe a request for a Formal Interpretation (FI) is best. They
ARE painful to get, but if you wait to make a PI, odds are it will be rejected without heavily documented substantiation that a problem actually exists. As you mentioned, if an "NPT" coupling
is required, it has been violated millions of times - mostly because no one makes them. And, if a problem actually exists, why wait four years to correct it? CMP14 has been one of the panels least likely to accept a PI simply because it sounds like a good idea. (Not that it hasn't)
The NEC is replete with unentended consequences. As I mentioned, the origin of Section 500.8(E) was from the introduction of Zones in 1999.
The FI would read something like:
In light of the requirements of Section 500.8(E) are simple couplings, where used, required to have NPT threads when installed between enclosures required to be explosionproof and their required seals?
If it is answered in the negative, the CMP is obligated to "clarify" the Code (in possibly several locations) in the next cycle and possibly issue a Tentative Interim Amendment (TIA).