I would imagine that putting tape over the crack would then introduce 2 more fine lines at each side of the tape that would need to be covered. Then he would have to add a coating over the tape to hide those tape edges, which would result in a mound on your flat ceiling. This could be blended in by making it wider, but it seems like it would add more problems than it solves. The thing with ceilings is that bright light often shines across them, so any variations in thickness looks worse than it does on a wall, where the light usually arrives from many angles. (Try shining a torch along a plaster wall to see how (un)flat they are.) If the 2 edges of adjoining plaster sheets are attached closely where there is timber above the join, the tape shouldn't be needed. It may be a case of redoing the fine cracks once more now the plaster sheets have settled.
