It just makes sense with the global idea that C- is for
atomic movements, and M- for group movement.
That might be an argument for users to bind it, but that in itself is not
an argument for Emacs to bind it by default. And it is certainly not an
argument to bind it to some specific key (which one?).
FWIW, it is an argument for that last point: C-b is backward a character,
and M-b a word; C-a is beginning of line, and M-a of sentence; it makes
perfect sense for C-n to be next line and M-n to be next paragraph. That
doesn't mean we should do it, but that's why M-n makes sense for it.
Davis