Why does C-M-o `split-line' leave trailing space on the old line?

[Alan Mackenzie]

Hello, Emacs.

C-M-o, on something like:

     foo bar baz
            ^
          point

will split the line into

     foo bar
             baz

, but will leave a trailing space on the first line.  This is in
accordance with its documentation in emacs.info, but this peculiarity
isn't mentioned in the doc string.

I find this trailing space irritating, as I frequently have to remove it
by hand.  What is the motivation for this trailing space, and do we
really want it?

-- 
Alan Mackenzie (Nuremberg, Germany).