Most password policies are dumb, but not for the reasons you might think. For one thing, password length matters far more than password complexity.
What Elmo said is also quite true. Scheduled rotation policies are appropriate in some high security environments where the user base understands actual password complexity and are dedicated to maintaining it, but in most environments they just weaken security. In most corporate environments, the only reason to change passwords is because a breach has occurred.
And to be completely blunt, most policies are wishful thinking. A randomly generated, 16-character password with uppercase, lowercase, and numbers has 95 bits of entropy. But when users are asked to create a password meeting those character and length requirements, they choose passwords that have far less entropy. You can't save users from themselves. If there's any sort of "logic" (read: "pattern") behind how you create your passwords, complexity requirements are moot. You're an easy mark because crackers are smart enough to think like you do.
http://arstechnica.com/security/2013/05/how-crackers-make-minced-meat-out-of-your-passwords/2/http://arstechnica.com/security/2013/03/how-i-became-a-password-cracker/