If I only use the system once a month, and you make the password expire every month, then every time I log in, I will have an expired password.
And if you fail to tie the password into something sensible so that I have a universal password for as many systems as possible, it will frustrate me to no end. It will be impossible for me to remember my ever-changing password, and so I will record it in a plain-text file on my hard drive, thus granting access to anyone who knows my Windows password, which should hopefully be only me, and is what you should have done in the first place.
And the frustration will be so bad that I will go gripe to my friends and they will become aware of my breach of sound security principles in recording my password on my hard drive. And those friends are online. So the world will know.
Just saying, is all.