The primary benefit of database slaves is to be able to offload database reads and dump style backups (eybackup) to this host. Neither of these occurs automatically on adding a slave but the slave database does automatically get added to your database.yml and there is a cookbook in the ey-cloud-recipes repository for managing the backups.
Expanding on this important issue a bit, while database failover is not currently a customer facing option it is available as manual option to EY Data Engineers. This is mainly because MySQL replication is both asynchronous and statement based which carries its own risks. In most cases the terminate/rebuild from snapshot option is the safest, and fastest method to bring your database back online. The primary area that is an exception to this is for pre-planned instance size or disk upgrades in environments with either very large disk volumes or a large number of server instances.