That's like asking how many ubuntu users run their account as root. Almost None. The default in both cases is a normal user with sudo access (requesting a password). I'm not even sure where to start if I wanted to log in as root (not just in a terminal).
I just tried logging in as root on my Mac from the graphical login, works fine! If you don't know your root passwd you can also 'sudo passwd'.
There's no advantage really, though, since the gksu style authentication popup system when you need admin privileges is very painless. I too doubt if many people run their Macs as root.
Maybe when you typed 'sudo passwd', which created the root password. Just as in Ubuntu the root user exists, the account is merely locked (see 'man passwd', -l option)
Apple describes how the Right Way here http://support.apple.com/kb/ht1528 but I'm sure I've never seen Directory Utility before. However, in the Edit menu they reference, it said 'Disable Root User', not Enable, so it's been activated somehow. Perhaps doing sudo passwd did it, though I don't recall setting it up through an unofficial method such as that on my Mac. I'm not really comfortable hacking around on MacOS since it's a bit different than Linux, being a BSD, and the extent of integration between traditional text file style configuration and Apple's GUI stuff is unclear to me.
I previously enabled it via Directory Utility, but disabled it again and tried it for you:
$ sudo passwd
Changing password for root.
New password:
Retype new password:
passwd: Unable to change the password for record root. Credential verification failed because account is disabled.
$
So there you have it: you couldn't have done it this way.
