Sorry for not following up on this one in time. I filed a JIRA (5651) and it seems user lookup is here to stay.

https://issues.apache.org/jira/browse/CASSANDRA-5651?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

On a related note, that column family is, by default, set up to have key cached only. It might be a good idea to have its row cached turned on if row cache is enabled.

Bao