I'm guilty of using l33t sp34k in the early 1980s. At least on the BBSes where I hung out, it came into use as a way to defeat profanity filters installed by the sysops. $#!T was a common construction.
It's funny seeing it these days -- unless you're using a monospaced font, most l33t sp34k doesn't look a whole lot like the "real" text it's supposed to be.