I had the same reaction when I saw that field -- it doesn't appear to be used.
In fact, I don't think it was ever used. Grepping some of the older releases (http://svn.civicrm.org/civicrm/branches/v2.0, http://svn.civicrm.org/civicrm/branches/v2.2), the field appears as part of the 2.1 schema upgrade; however, even then it's not used in any logic.
My suspicion is that the field is more aspirational. (Theory: One person designed the cache layer and included
expired_date in the schema because it's standard in caching systems; then another person implemented the cache logic but ignored that part of the design.)
I wish the cache supported expiration/TTL, so I'm not anxious to remove the field. It would be even better if someone finished TTL support.