There are a number of reasons why replacing Athens by something like Shibboleth is a good idea. Perhaps the most important is that it reduces by one the number of distinct userids and passwords our users have to remember. The fact that we will no longer have to manage the Athens id/password set is also worthwhile.
Athens is a problem for SPs too – it is UK-specific and so requires implementation work on their side. They, and JISC, also have to pay a significant sum to EduServ for the Athens service.
Athens lacks the privacy-preserving options of Shibboleth.
Shibboleth was originally designed with access control for electronic resources in mind, but it is important to remember that this is far from the only thing it can do.