We as individuals are a composit of traits, preferences, past experiences, aspirations, etc. etc., like pieces of a puzzle. A person can't really be judged by any single piece of the puzzle, but together they paint a picture that tells us if a person is to be taken seriously, or if the person is a doofus. Grammar is an important piece of the puzzle, as it says much about you. The occasional slip-up should be of no concern, but a pattern of bad grammar tells me that somebody is either lazy or ignorant. Either way, they loose credibility.