It can be very usefull to have an abillity to switch jira 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 Jira in read-only mode: data can be searched and viewed, but not modified. This could decrease downtime during upgrade, too.
Several testers of our team really suffer, if they havn't loved bug-tracking system online.
There is an issue JRA-1924 which seems to be forgotten for a long time. It suggests very similar functionality and there are ten votes for it.