>> My own experience is likewise that I can't imagine ever using a prefix 
>> argument with split-window-{right,below}.
>
> It doesn’t even work (correctly).
>
> C-u -4 C-x 2
>
> ⇒ the resulting bottom window displays slightly more than two lines.
>
> (Disclaimer: I’m using a non-nil line-spacing; but if I set it to nil, 
> then the new window displays slightly less than three lines.)
>

That's the expected behavior, because the mode line is part of the height 
of a window.  So with emacs -Q, C-u -4 C-x 2 creates a window that is 4 
lines tall, 3 lines for the window body and 1 line for the mode line.