You are absolutely right. Someone with a bee in their bonnet can give you a lousy review out of pure spite. I have an rental mainly let through Airbnb. I have been letting since last December. I had seventeen 5 star reviews, but one 2 star review from a petty guest because I couldn't let him have early check in, since someone checked out the same day. Of course this drastically affected my rating and, a huge fault with Airbnb, they don't support hosts, said it couldn't be removed because it was the "subjective opinion of a guest". Utter rubbish, it was a total lie, but as Nakhonandy says it can seriously affect your business.