Suppose I want to use GnuPG on a GNU/Linux laptop where I've encrypted all filesystems (except /boot) with LUKS/dm-crypt. Backups are encrypted client-side and stored on a physically secure server. Given that my files are already encrypted-at-rest, is it worth the trouble to passphrase-encrypt my private key as GnuPG suggests I do?
If so, describe the exploit(s) that would enable an attacker to steal my private key file but not my passphrase. How practical are such exploits compared to those that LUKS already defends against (e.g. seizing the laptop from my luggage) and those that would defeat both LUKS and the GnuPG passphrase (e.g. an evil maid or a remote code execution vuln, both of which could get a keylogger installed)?