1337 aesthetics are particular and requires extra interpretation to process. This makes it hard for people who don’t get it to read, which is why subcultures like it. It is also hard to remember, which is most why password users would rather not use it.

Free letter-number and letter-symbol substitution combinatorially expands the possible number of ways of saying the same word, so a subculture discourse becomes less search-engine searchable: many people are spelling “owned” in a dozen or a hundred slightly different ways, occasionally using a consistent misspelling based on their specific message board / forum / niche. For an insular or underground subculture, this is a feature of 1337 - it opens up language to regional inflection and accent, which brings with it the potential for more shibboleth’s that expose outsiders. For password memorizers, of course, tiny seemingly-insignificant variations in the basic sense of a word or phrase are pure poison - they exploit the gap between what a symbol string is remembered for (its sense) and what it actually is, leading to more forgetting and failure.

I’ll go on at some other time, but you get the idea. I think IM often gets represented purely as a kind of subversive misspelling, the way people discuss the politics of ebonics grammar patterns. Efficiency is also part of the picture, but I wonder whether or not privacy and security aren’t at least as important - and in that sense, whether the culture of 13375p34|< and corporate mainframe security best practices might be mirrors rather than antitheses.