AGAIN...Phillip is correct
The ORIGINAL reason to change passwords often was to correct admin mistakes of not removing old users (ex-employees). ALSO....remember the movie War Games w/M. Broaderick....they used a shared password. So changing the password every so often protected them when users became ex-users.
Point is....we don't need to do this anymore. Our security has evolved into a sophisticated system where it is easy to remove employees from the user profile.
IMHO....changing passwords is another excuse by systems admins to justify their jobs. Lame.