C-z and C-x C-z are bound to suspend-frame

["paul r" <paul.r.ml@gmail.com>]

I notice today that on emacs -Q, both C-z and C-x C-z are bound to
suspend-frame.
Since a long time now, I have in my .emacs
   (global-set-key "\C-z" nil)
And when needed (not often to be honest) I use C-x C-z for suspend-frame

I use C-z as a prefix for emacs-wide personal bindings. I read
somewhere I should use C-c but many modes already use it for
mode-specific bindings, and I feel comfortable with this simple rule :
   C-x is for emacs default bindings
   C-z is my own space for global emacs bindings, like mpd-next-song
or whatever should be accessible from everywhere
   C-b is mode-specific, so any mode is totally free to use this space
for its needs

That way, I ensure my global bindings will never make collision with
emacs default global bindings, nor with mode-specific bindings. But
this is a bit hackish.

Two questions :
 - is there a reason why C-z is kept bound on suspend-frame along with C-x C-z ?
 - generally, how do you manage your personal bindings ?

-- Paul