/etc/master.passwd was added in 1bf9d5d9518e with a toor account (and disabled by default in 9f808f21bf4b). It originally used a different shell (sh) which provided a backup in case the main root account's shell (csh) was broken. Root's default shell is now sh so toor no longer serves this purpose.
Even if disabled by default the existence of an extra superuser account may be alarming to administrators and to security auditing systems. Just remove it by default. Sysadmins who wish to have a second superuser account can add one.