For a very long time, fixing MySQL slaves was a tedious task. Slaves died, replication fails, tables would get out of sync and you’d have to import all the (consistant) data from the master database. Depending on the importance of the slave, this could mean downtime for your master while exporting.
MySQLdump is usually the application one uses to execute this task, locking the tables while exporting. For me, having a database readonly, is downtime. However you look at it, if your website is not 100% useable, it is experiencing downtime.