Why do you think Blog posts that have spelling, grammar and punctuation mistakes reflect poorly on the Author?

Because people don't consider the way reader reads your blog. Writing badly on your blog can affect you in a bad way and makes you look dumber. Even if you don't expect that much people to read it, a future business man can look at it and can get you a job or opportunity to get a chance in life. If he see your bad hand writing. You may not get that job and you will regret it. Writing good on your blog can make positive comments and thoughts about you.