[equivrel]

Don't forget that passphrases are also have security uses other than authentication: symmetric encryption of private keys, disk/file encryption, etc. where their use is pretty much unavoidable. Obviously a very long randomly-generated encryption key saved to a usb stick is more resistant to brute force than a memorable passphrase, but you still want to symmetrically encrypt it with a good passphrase to keep it (relatively) secure in case it gets into the wrong hands.