Auxiliary keychains like that automatically re-lock themselves after 15 minutes iirc. This also allows you to create disk image passwords of arbitrary (military? FIPS?) complexity without having to memorize them. If someone happens to steal the DMG file, they have no way of knowing the password is stored in some auxiliary keychain somewhere that has what might be a much easier to break password.
But if someone gains access to your computer,
all of those super-strong passwords go by the boards in favor of the possibly weaker keychain password.
Bottom line is that there
must be one password that is both super-strong and unforgettable at the same time.
Catch-22 again?
