If you compromise the machine, you can attack anything on that machine. If the keys are in the home dir on the network, you can attack the network. If the keys

If you compromise the machine, you can attack anything on that machine. If the keys are in the home dir on the network, you can attack the network. If the keys are in the home dir on unencrypted local disk, you can attack the disk. If the keys are in a user namespace, you can attack the user namespace (which includes processes and mounts). If the keys are in a thumbdrive and in a keyring, you can only attack two things: 1) the user namespace, 2) the thumbdrive's mounted contents while it is inserted _and unlocked_. This limits the scope of attacks. When the keys in the keyring expire ("-t life" option to ssh-agent), you can't even attack that.