for me, i think it comes down to what i am used to. i am "literate", i read a lot, and what i read is generally legible, well-formed (in an "academic" sense), and contains punctuation.
as a consequence of this (i suspect), the written english i have the most trouble with is originated by native english speakers who have never learned how to use punctuation. gigantic run-on sentences are mostly illegible to me. i have to follow-up, ask questions, etc. (i also have problems with spoken english, when the speaker's accent differs significantly from my own, probably due to my lack of training in other languages, but also possibly because i am going deaf from listening to too much metal, which really only sounds good if it is LOUD
).
at the same time, i have had people criticize me for not capitalizing the first letters of my sentences (and proper nouns). i plead guilty (i am just too lazy to use the shift-key, and WAY too lazy to go back through some text i have written and make sure that i did so correctly).
i suspect this is all just "human nature". we do what we are trained to do (punctuate/not, capitalize/not, interact with people who "talk funny", etc). and in practice, language is a pretty informal/mutable process to begin with.