Therein lies the potential merit here...
If Google paid to get Yelp content on Google Search results, then there's clearly some value in that content. If Google use that to promote their own reviews service, and eventually end up with enough content of their own that they don't need Yelp, then they have two options (in the eyes of anti-monopoly regulators):
1) Show their own reviews, but also, and in approximately equal measure, promote other companies reviews (and seeing as Yelp leads the market in that area, that means showing a lot of Yelp on search results)
2) Go to court and duke it out there
Turn this around to your own view point: Lets say you've got a blog all about fishing. I happen to run the biggest fishing related website on the Internet. I come to you and ask to put your content on my site in certain situations (to fill in some gaps in my own content) - and you agree. However, when placing your content on my site, I advertise to my users that they could write up stuff like this on my website's knowledgebase wiki (and earn points which could win prizes).
A couple of years go by and my wiki is looking pretty awesome. I've got nearly all the same information on it as you've got on your blog, so I terminate our agreement. Suddenly, you don't get income from me, and don't get anywhere near the levels of traffic you used to, so get less advertising income too. Wouldn't you look to try to get some sort of redress against me for my somewhat underhanded business practices?
Turn this out a third way (maybe a bit more tenuous, but here goes)... Let's say your favourite political party is blue. Would the world be a better place if blue took over the entire country and apart from maybe a couple of seats, entirely wiped out the red, yellow and green parties? Would it really be better in a few years time?