Thank you, Nick, for your reply. You have introduced other two interesting point of view: tightness and aesthetics.
I use to wear my watches not so tight and close to the back of the hand (in any case after the head of the ulna). This habit leads to a potential conflict with the wrist. That’s the reason why I’m extremely interested in a timepiece comfort. Correct me if I’m wrong, but I also think that who wears his wristwatch before the head of the ulna, should have less comfort issues and consequently should be less sensitive to the case back shape.
On the aesthetical point of view I think that a flat case back could look more sporty, strong or even rough, while a domed one could inspire a certain refinement. A flat case back could be associated to modern lines, while on the contrary a domed one is frequently associated to traditional design. At last, I agree that domed backs reduces the apparent height of the watch, since they can be chosen to accommodate a higher movement in a given case.