I just got through reading the UseRealWords article, and have no idea what "h4x0r353" means, and, since Google didn't turn up but two articles (one was the JavaRanch reference and the other was a dead link), I'm now wildly curious. What does this mean?
So called 'cool people' aka '12 year olds' sometimes talk in 'l33t speek', which is just a way of making it harder to understand each other. Swap E for 3 (cos they look similar, 4's and A's, 1's and L's or i's and provide a few more steps like typing errors and abbreviations to make yourself misunderstood at every opportunity.
Hance the reason we discourage it on a teaching site
h4x0r353 == hackerese, or the language of Hackers^H^H^H^H^H^H^H 12 year olds who just discovered this internet thing.