While OS X is generally a "single-user" system, having . in your PATH in any UNIX is considered risky, because someone could place a dangerous executable named something like ls in some random directory on the system. Once you cd into that directory, when you do an ls to take a look maybe your home directory is going to be wiped out, etc. In the case of OS X there's probably not a "someone" that would do this like they might on a multi-user UNIX, but it's just "best practice". Typing ./ before commands isn't too hard, and you're likely going to be hitting up arrow to get to the command in your history more often than manually typing it.
It's a bit disappointing that this recommendation would be published. Hopefully no one breaks into your system, but if they got in via some under-privileged account, but dropped an executable somewhere that you (using a privileged account) might run then a pretty low-impact exploit becomes much more dangerous.
-Lee