That makes sense. Now I understand why I saw that very same thing on a rigid conduit run that was installed in a very old building. I seem to remember it was 3", so im sure that would have been a very expensive erickson.Close nipples actually have taper toward the center.
There is UL listed running thread conduit that you can cut a hunk off of. Picoma (Beck) makes some. I'm just not sure how you compliantly use it.
I think the main objection to running threads came in the pre-EMT days, when installers would run a ton of thread on a pipe to spin a coupling on the whole way, and then center it up when the next piece was butted up to it, to avoid using an Erickson coupling.