Moving to hibernate would give us much more efficient searching of users (not having to load them all into memory) than we get currently with OSUser. Plus we wouldn't have to get people to migrate manually whenever they wanted to use LDAP.

In the past we have advised all Confluence customers to manually upgrade their (non-LDAP!) OS-User based installations to Atlassian-User based, because Atlassian-User is way faster when it comes to user-related operations like searching. With Confluence 2.7, we automate this process - on restart of Confluence a (non-ldap!) OS-User based repository automatically will get upgraded to Atlassian-User.

Check the background information in the description of the (now unneeded) manual process: http://confluence.atlassian.com/display/DOC/How+to+Improve+User+Search+Performance