Yes, readers and writers, every review is important. In a way authors may thank Amazon for forcing their readers to write at least something, when they rate a book, because: every review is important.
Imagine: nothing is more frustrating than a bad rating without a review, because there is no explanation. It's just like a dead fish right into the face of the author. So, even a few words to why this reticular book wasn't a 5-star read is helpful.
Of course, writing "I hated it." Isn't that helpful, but at least it does show an emotional reaction to this book, which might be helpful for the author if he or she thinks about it. Every author is aware about specific plot twists, or character traits that might put people off.
If you, dear reader, want to keep it short and still be helpful, point out at least one thing that did put you off. It's not that hard. Sometimes only a few words may make the difference. If you hated it, tell us why: "I hated it, because to me (yes, 'to me' is very important if you want to be respectful) the female lead was stupid." That's something every author can accept, because they can't change what you like but now they can respect your rating. It wasn't that hard, was it? If you add another explanation every author will be grateful - and if they aren't just don't buy their books.
No one has to write an essay about a book they didn't like, but a short explanation is not only helpful to an author, but to fellow readers as well, because they might not buy the book because of your explanation. And no, dear authors, that is not a bad thing. Reviews preventing purchases can be a good thing, because it decreases bad ratings. In the end you want only people to buy your book who enjoy it and feel compelled to purchase more of your books. Because readers who enjoy your books are way better than those who hate them and purchase your books only once.
So, yes, reviews with bad ratings are even more important than reviews with good ratings. These reviews will alert people who will not enjoy your book and prevent them from buying it. Which means that the amount of bad ratings will decrease over time.
Long story short: every review is important.