Think of reviews like a handshake. If someone walked up to you at a function and extended their arm gesturing to shake your hand, it would seem strange, if not rude, to ignore it and leave the person hanging.
Leaving a review not responded to, is akin to leaving a person standing there with their hand out, unmet by yours.
Here is why you should respond to both positive and negative reviews.
When you reply to positive reviews, you are saying thank you for taking the time out of your life to add value to our business.
You are extending their positive experience to post purchase, and depending on your response, probably significantly increasing or at the very least reinforcing the likelihood of positive word of mouth.
Replying to negative reviews is arguably even more important.
You see, when you reply to negative reviews your audience is not really the person who wrote the review itself.
Your audience is the people that will be reading your response in the future.
It is a great opportunity for you to clarify the truth, share your side of the story, and demonstrate why your product or service was never going to be right for this person.