Yes, Mac OS X allows use of
sudo when the root account has not been activated. Administrators will be put in
/etc/sudoers when their account is created; you can edit it manually to add or remove users the old-fashioned way as well.
The reason is mainly security. As any *nix veteran will tell you, logging in as
root when you don't have to is A Bad Idea. As
root has read, write and execute permissions for everything, a few errant keystrokes can make a real mess of the computer. This, of course, makes it a favourite target of hackers and malware authors, too. Presumably Apple deactivates root by default to discourage users from logging in as it (perhaps they were used to running as Administrator on Windows, although this is just as foolish as logging in as
root) as well as making it more difficult for someone with ill intent to access the root account (if you enable it by default, you either need to use a default password (which can easily be guessed or figured out) or get the user to choose a password (users generally choose crap passwords)). The root user does, however, still exist on Mac OS X, even when it's disabled - all that does is prevent logging in as
ssh -l root and so on). As you might expect, many processes are run as
root, including these launched with sudo.