Good point. I've toned down my comment because that root password would be getting typed in less often.
An attacker might still bring an evil-su in addition to an evil-sudo, though. And even if you're logging into that root user only in an another tty, it seems like an unnecessary risk to share the password with LUKS.
You actually tend to never use the root password on a modern workstation -- just sudo. The situations where you have to use a root password are usually if something has gone wrong and you have to log in via tty.
An attacker might still bring an evil-su in addition to an evil-sudo, though. And even if you're logging into that root user only in an another tty, it seems like an unnecessary risk to share the password with LUKS.