Crackers and Script kiddys for sure, never hackers, they don't need to pretend
It was used a lot as you said, back in the 80's, particularly on Usenet. The idea was that the language would be scrambled so that scripts and search engines could not detect keywords, so certain things could be communicated without being scanned and either blocked or detected. As a "human" code, it's pretty poor, because you can read it easily, but it's much harder for a script to decipher - or it was back then. ROT13 was another one, but again, a very simple code to break. Then pretty good privacy came out as freeware, a really solid key-based, on-the-fly encryption system that was "government proof" and leet speak was no more. The US gov tried to ban PGP, they tried to prosecute the developer and stop it's distribution, but once it had gone wild and started to pop up on Russian and Chinese servers, they were on a loser. Any real hacker used PGP and leet speak was left to the kids.
The interesting thing about Bikers paragraph at the beginning, is what it tells us about how our brains work. We dont actually read every letter, we scan for visual patterns. Reading is all about recognising visual shapes of words, rather than deciphering an assembly of characters and spaces. Even if the actual characters and spaces are wrong, providing they trigger the right visual cues, providing the shapes of words look close enough, then we can read it.
Airdoccng to a rsrechecah at Cadmrbige Ustvieriny, it deson't matetr in waht oedrr the lterets in a wrod are, the olny iemorptnt tinhg is that the fsrit and last letter msut be at the rgiht palce. The rest can be a ttoal mses and you can siltl read it wtuhoit ploberm. Tihs is bseauce the hmuan mnid does not read evrey letetr by itlsef, but the word as a wlhoe.