It can be very usefull to have an abillity to switch confluence in read-only mode. During our last problems with database, which is hosted several application including Confluence and Jira, all mentioned services was unavailable. It is desirable to be able to run some kind of backup instance of Confluence in read-only mode: data can be searched and viewed, but not modified. This could decrease downtime during upgrade, too.
Several members of our team really suffer, if they havn't loved wiki online.