George mate, who misses scrim off? It's only the very basic bodgers and I suggest if they don't have the knowledge to put scrim on, they probably wouldn't have the skill to lay plaster on a ceiling and if they did get it on, you'd see a lot more issues up there than a hairline crack and the customer would be screaming about the finish, the cats faces, the spider holes left in the corners, the sxxt on the walls and floor and all those types of issue. (starting to think crack is being used).
I did the best I could do on the above job and something caused 12ft of fibre tape to snap apart right along its length. My example is a 100yr old terrace so it's had a V long time to settle, etc, no-one went in the loft and the boards were fixed across the joists. It had the mist coat and an hour or two later the thing snapped in one go with a sharp 'thwack' sound.
Having given it a lot of thought, I think it was the scrim around the ceiling perimiter that has helped this. Now bear with me. The whole room was re-skimmed with scrim in all the appropriate places. So if there has been thermal movment in the ceiling, with the outer perimiter of the ceiling boards being fixed so to speak, if the ceiling needs to move, it will break the weakest joint in the boards and that must have been across the middle.
