>
> I hadn't realised until now, but split-window-{right,below} actually 
> already have a meaning for a negative prefix argument.
>

Indeed.

>
> I don't know how we decide whether to usurp that with having it mean the 
> command should affect the root window.
>

Note that the C-x w | and C-x w - that Stefan and I (at least) use solves 
that problem by circumventing it (and by putting the command on a key that 
is related to its meaning).

>
> My own experience is likewise that I can't imagine ever using a prefix 
> argument with split-window-{right,below}.
>

We agree on that point 😃