Andy, I don't think what you say is quite true. Unless you have a memory management which _effectively_ disables other modules from fiddling with "hidden" data = a hardware which you don't have in '51, there is no reason for hiding the symbols (other than you don't want to disclose them not to leak out information the internal whereabouts, as in closed source software).

Sure - the actual code can change any memory location whenever it wants to.

The aim of not publishing all the symbols isn't to keep the code from doing this (which would be a bug if it is not intentional anyway), but to keep the programmer away from the temptation of wildly messing with variables all over the place.

See the "accumulate and fire" antipattern.

http://en.wikipedia.org/wiki/Accumulate_and_fire