I think first, we need to look into what su and sudo actually are
su - stands for Substitute User. You use this to switch to a shell as another user using that user's password. Commonly used with root.
Does not require a password when executed as root.
sudo - allows a permitted user to execute a specified command as another user. Also commonly used with root. However, this allows you to specifically manage what commands may be executed as another use. (For instance, you could give a user the ability to run an init.d script but nothing else.)
Note, you can always run
sudo su or
sudo -i and that will give you a root shell. However, no root password means no logging in directly as root... which means no one can break into that user.
EDIT: so maybe this answer your looking for is:
not having a root password forces you to use
sudo, which in turn naturally aligns you with the
sudo philosophy which suggests you to enforce greater control over the actions run as root.