Any text with mistakes in it reflects badly on the author – whether it's a manuscript for a literary agent or publishing house, a CV for a potential new boss, or an advert for a magazine. You can't guarantee that the reader will like it, but you can guarantee that they will put it to the bottom of the pile or to the back of their mind if it's full of errors. I know this is a fact – I've done it myself when considering articles for publication and CVs.