Is there any reason NOT to use File Vault?
Is there any reason NOT to turn on file vault?
MacBook Pro Retina
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If you forget your password you migth have to erase your drive, and losing what you have saved on your disk,
But other than that no, not really, Its advice if you want top protect your data.
And you can make sure to have a time machine backup that's not encrypted, but stored in a safe location, to make sure you dont lose any data.
Use FileVault to encrypt the startup disk on your Mac - Apple Support