When a customer fills out a review they usually put in key word rich terms and phrases. So not only does this help you rank for a main keyword but also sub keywords you might not being found on now at all. this will only bring you more customers. For example if your a plumber you want to rank for keyword plumber. But if a lot of reviews keep saying you fixed a "broken sink" you will be found when people search for fix a broken sink. Many of or customers have gained tons of new customers for being found on keywords they never targeted.