Faulty spelling and grammar are to some extent just a regrettable lack of skill, like being bad with numbers or bad at reading maps. But they can also show a lack of consideration for one's readers, namely when they force readers to do extra work to figure out what the writer meant to say. Even if you care nothing for proprieties, you should accept responsibility for making your meaning clear, and not put your readers to the trouble of working it out because you could not be bothered to do it yourself. (Addendum: The pronoun "you" here does not refer to the original poster.)