Yes, reviews (even negative ones) posted broadly are a huge help to writers. I've heard that Amazon's algorithm places a heavier emphasis on number of reviews than the actual star ratings, although I'm not sure if that's true.
On the quality of reviews, look at it this way, by reading the old ones, you can see how much better you're getting at it. I'm sure I'd be appalled if I were brave enough to read my early attempts at novel writing, but I'm not.
10 years ago
Yes, some book promotion sites only advertise a book if it ha a certain number of reviews so every review helps the writer, even the bad ones.
On the quality of reviews, look at it this way, by reading the old ones, you can see how much better you're getting at it. I'm sure I'd be appalled if I were brave enough to read my early attempts at novel writing, but I'm not.