Root as Primary Login: Why Not?

A user writes, "I help moderate a forum dealing with Mac OS X, and I'm having an awful time convincing a fair portion of our readers that logging in as root all the time is a Really Bad Idea. Worse, though, are the ones who try to convince others to log in as root all the time, claiming it's 'more Mac-OS-9-like,' or saying 'it's not really more insecure,' or even that 'a firewall should deter hackers pretty well.' I know all the standard arguments, but they're not working out. Does anyone here have some real-world anecdotes that I can point to?"