I'm not disputing that. I understand it and I consciously make allowances for it. But my point, and the reason I started this thread, is that not everyone performs that analysis consciously. Language is such a basic component of human nature--after all it's one of the things that makes us human--that a large part of our reaction to someone is based on the way they use the ancient technology of language.
People don't all have control over how they react to sloppy writing, but people do have control over whether they write sloppily.
As for laziness and whether it's "important," Pareto's Law is fine: just put in ten percent of the effort and eliminate ninety percent of the annoying errors. As I have said several times--in this thread alone--it only takes a fraction of a second to use the Shift key diligently, but it makes writing enormously easier to read. E.g., to write "MBA" instead of "mba", which I stumbled over and had to go back and read twice. My brain's first guess was to interpret it as a misspelled word, not an acronym.
Every tenth of a second you save by writing too fast and too sloppy probably wastes a full second of each reader's time. Multiply that by the number of people whom you hope will read your post. In a year of posting you've wasted so much of the membership's time that they never got around to reading some other poor guy's posts. So it's a discourtesy to everyone.