So one misspelled or overlooked grammatical error makes an entire piece unintelligent? That is a pin headed view IMO.
One misspelled word or grammatical error indicates that the writer either does not care about doing his best work or that he was not educated well enough to work for me.
I'm not referring to the occasional typo or grammatical error. I, too make occasional typos and an occasional grammatical error, especially if I get careless when using "cut" and "paste" features on the computer to move text around. However, in my view, any adult whose native language is English and who doesn't know--or maybe doesn't care--about the difference between "there," "their" and "they're" is a pinhead.