I think this feature should be added to the game because of a reason I have not seen anyone mention before.
I always play osu without any interface nor scoreboard in single player as this is less distracting. I also participate in 1v1 tournaments where Team Mode is often set to Head To Head. Having no option to disable the ranking makes me perform considerably worse in such tournaments as the ranking stands out quite a bit (even more so with dimmed background.) Tournaments should not have to force the use of interface onto players which normally play without the extra distraction.
The argument "why play multiplayer if you dont want to see your rank" does not apply here since tournament matches have to be played in a multiplayer lobby. "Just use a piece of duct tape to hide it" does also not work as in ctb it will partly hide your catcher when moving to the right.