Imho I think they are very valuable. I know the owner of a restaurant that told me he will carefully go through every Yelp review and call the one that lower his average of 4+ stars.
He says many people search on their phone where to eat, and they use mainly Yelp, judging with stars first and reading later the reviews.

One unsatisfied client with one bad review can create a lot of damage.

The important thing about Yelp is that they build the business around it making a community of viewers & reviewers. You can read it here http://pando.com/2013/12/09/when-yelp-is-good-enough-can-anything-else-beat-it/ how the network effect it had was almost monopolic.

Here are some stories about Yelp (the one I have follow the most):

http://www.inc.com/jill-krasny/now-this-is-how-you-handle-a-bad-yelp-review.html

http://www.fastcompany.com/3027249/lessons-learned/how-yelp-encourages-users-to-write-more-thoughtful-reviews-even-on-mobile

http://www.fastcompany.com/3033942/fast-feed/this-new-york-hipster-hotel-fines-you-500-for-bad-yelp-reviews-and-yelpers-are-pis

Also remember how now everyone got used to use reviews (Amazon, Airbnb, Uber,etc) so they are not going anywhere. They will actually get smarter (I'm writing a post on this)