Hello,
I think there might be a misunderstanding there.
I read here:
http://www.borg.com/~jglatt/tech/midispec/ctloff.htm
that the "All Controllers Off" message resets controllers to some default values.
That may probably be different than zero for some of the controllers. It also is different from leaving the controller value where it is.
In the link I posted above, those which are reset to zero seem to make sense.
Maybe we should come up with a list of targeted controllers if we think that the link above does list too much or too few controllers.
Duncan, probably the "can of worms" expression there is a good one 
Regards,
Xavier