Ubuntu doesn't set LC_ALL, why?

If you run locale on an Ubuntu installation you get something like:

LANG=en_US.utf8
LANGUAGE=
LC_CTYPE="en_US.utf8"
LC_NUMERIC="en_US.utf8"
LC_TIME="en_US.utf8"
LC_COLLATE="en_US.utf8"
LC_MONETARY="en_US.utf8"
LC_MESSAGES="en_US.utf8"
LC_PAPER="en_US.utf8"
LC_NAME="en_US.utf8"
LC_ADDRESS="en_US.utf8"
LC_TELEPHONE="en_US.utf8"
LC_MEASUREMENT="en_US.utf8"
LC_IDENTIFICATION="en_US.utf8"
LC_ALL=

How come LC_ALL is unset? I know how to set it, but why doesn't Ubuntu set it automatically, like with the other LC_'s?

If LC_ALL is set it overrides the values of all the other LC_ variables. Hence setting it by default would have the same effect for your locale settings as setting all the LC variables but would make it more difficult to change only some of the values.