In my opinion, small grammar mistakes can be overlooked if the overall insight of the work is impressive. If the author's writing is engaging and unique, then a faux pas or two would probably go unnoticed. We all make grammar mistakes from time to time, whether we are communicating in our native language or another. I wouldn't feel comfortable hiring someone to write something for me that wasn't going to write it in their native language. It raises the probability that I will get my money's worth.