Brent Simmons pays attention to the details and gets right to the point:
If your writing — in tweets and especially on your blog and product pages — is full of misspellings and improper capitalization and other errors, I will lose trust in you and your product. If you’re careless with language, are you also careless with software development?
I make misspellings and other errors more often than I’d like. Everybody makes these mistakes. Yet, I think it’s important to put as much care into finding errors in language at least as much as finding errors in code.
(The crux of his post is something that I was instructed to avoid over and over again in middle school. I can relate.)