the reason we have recognizable grammar guides in English (they're decentralized but recognizable) is because, in storytelling, grammar is necessary for ease of reading, which contributes to immersion. in business writing, following an established grammar style shows that the writer is serious. those are, however, the only two reasons "correct" grammar is necessary. in casual communication - including forums, IRC, and message boards - 100% accurate grammar conveys a tone that isn't always appropriate
If someone speaks like this in a casual environment, complete with terminal punctuation, I assume that they're either very old or take themselves far too seriously to be any fun.
so in circumstances like these, we eschew capitals and terminal punctuation in order to get across the feeling of casual conversation. however, other parts of grammar are necessary - for example, you can't expect people to instantly understand a run-on sentence, so you should avoid making them or risk upsetting the pacing of the conversation. people who don't understand this (and grammatical prescriptivists who shoot down "bad" grammar when the communication is otherwise clear) are genuinely not worth talking to because they do not comprehend what language is for
but, yes, in a story, absolutely get some prereaders and editors if your story is serious 'cause of the immersion thing I mentioned