Not always, but the worst errors are in the wrong use of grammar. The meaning of a sentence can be changed and a reader may have to go back and reread the preceding sentences and then think "well, he must have meant...". If there are too many spelling and/or grammar errors it becomes difficult for some readers (me, for instance) to even follow the text. One who uses sloppy grammar probably does not think so neatly, either.
The readers are not telepaths. Correctness and clarity of written language on the forum is essential for the writer's ideas to be communicated in unchanged form. If the reader cannot comprehend the writer's meaning due to spelling and grammar errors, then the post was wasted space. If the reader obtains an incorrect or incomplete understanding of the writer's ideas due to spelling and grammar errors, then the post was wasted.
It's only different in degree to making a post in a language other than English, and then wondering why very few people respond to the ideas contained therein.
Besides, spelling errors and improper grammar look bad, are inconsiderate of the reader's time (it's enough work as it is to read the post and try to understand the idea, but to have to decipher the post before doing either?), and are something we would expect of an uneducated or illiterate Democrat. But I repeat myself...