For me personally, it's about the effort put into communication. Using poor grammar and spelling makes the author seem lazy or even arrogant, as though their me

For me personally, it's about the effort put into communication. Using poor grammar and spelling makes the author seem lazy or even arrogant, as though their message were so important that the recipient will not mind spending extra effort trying to understand it. I think I would not mind informality by itself as long as the thought behind the text is clearly communicated, to the best of the writer's ability. However, I suspect that better thought-out communication naturally leads to a more formal style of writing, which might include using the proper forms of address.