The reason that spelling and grammar matters to some people is that these people view the gift of language as more than a mere means of communication. Language, in the hands of one, has the ability to pass simple messages along to another. In the hands of another, it can convey abstract ideas in an understandable fashion. There is also an undeniable aesthetic perspective to this, as an elegant sentence structure adds a bit of extra authority and can even make the reading more enjoyable.
Despite using a second language, in a forum like this where the subjects include aesthetics, technicalities and sometimes even philosophy, I usually take care to express myself as correctly as possible because these subjects demand it. Language and music aren't that different, as is shown by their collision in poetry.