V-Sync is wildly inefficient and leads to output delay to the degree of a few frames. In competitive FPS this output leads to overcompensation of aim because you don't see the output quickly enough. It also decreases FPS significantly which in turn means that - even if your FPS is higher than your refresh rate - your game's processed output often does not match the exact refresh of the screen, giving less smooth and still "delayed" displayed frames. For some reason, V-Sync also gives a mouse smoothing effect but I have no explanation for that.

Games that have V-Sync and Mouse Acceleration options on Mac are overridden by the system. I tested most of this in CS:GO since these settings are very important. Disabling Mouse Acceleration in the game settings is immediately reversed, as is disabling and enabling "raw mouse input". The same goes for V-Sync. I was about to disable Mac's forced V-Sync using dev tools and a custom program somebody made disabled mouse acceleration. Still, it shouldn't take such effort for 2 simple settings.

tldr: Screen tearing is worth it in games where high performance is necessary.