OS X Mavericks Guest Mode with Filevault → Safari flickers + crashes
The Guest account only appears on the FileVault 2 pre-boot login screen if iCloud's Find My Mac is enabled. It's essentially a way to encourage someone to get online from that laptop without having to unlock the encryption. Once online, the laptop phones home to the Find My Mac service immediately with its current IP and location information. 

Long story short, the Safari-Only Mode is not really meant for everyday use. It's meant to be a trap for thieves.