Why should apt-key output not be parsed?

From man apt-key:

DESCRIPTION
   apt-key is used to manage the list of keys used by apt to authenticate
   packages. Packages which have been authenticated using these keys will
   be considered trusted.
   Note that if usage of apt-key is desired the additional installation of
   the GNU Privacy Guard suite (packaged in gnupg) is required. For this
   reason alone the programmatic usage (especially in package maintainer
   scripts!) is strongly discouraged. Further more the output format of all
   commands is undefined and can and does change whenever the underlying
   commands change.  apt-key will try to detect such usage and generates
   warnings on stderr in these cases.