Informal social rules, although they seem arbitrary when you examine them closely, do serve an important purpose. A man wearing clean clothes is communicating something to the world around him. He took the time to buy those clothes, to wash them and fold them, to put them on before he left the house. "Hello," his garments say to society at large, "I am NOT here to murder you!"
So why is it that a person's writing is not subjected to the same kind of scrutiny as their behavior on the street? Why don't we look at bad writing and see it as suspicious, too?
And I'm not talking about little stuff. If you make the occasional spelling error, forget a punctuation mark, or even make a grammatical error, that's one thing. People make mistakes, and writing is a skill that not everyone has.
But if you write a paragraph's worth of information without a single capital letter or period, all your thoughts separated by ellipses, then you need to be put on some kind of watch list. Just like a man babbling to himself incoherently, that sort of writing shows a complete disregard for any human life around the speaker. It is nothing less than the telltale sign of a broken, tattered mind.*
*See also: My work inbox.