Yelp and a couple other companies forced in Google to a Congressional hearing where senators attacked Google over having too much power over other sites. Yelp gets a SIGNIFICANT amount of its traffic from google searches. So if Google arbitrarily decided to promote its own restaurant reviews from Zagat after a user search, this would crush Yelp's traffic.
But yes, I agree that it's a problem that most websites face, not just Yelp.
There are lots of companies that are far more of a destination than yelp -- the obvious contrast is facebook. I'd bet very little of their traffic is google search driven. Weaning themselves from dependence on Google should be one of the top goals for any web company.
[1] http://techcrunch.com/2011/09/21/stoppelman-75-of-yelps-traf...
Instead of typing yelp.com (or was it yelp.net? www.yelp.biz? http://www.yelp.info?) I can just type "yelp" and get where I'm going without having to remember the exact url and without getting an annoying error page if I make a typo. I suspect for many users even in other browsers the search box gets much more use than the URL box.
I think Google should have just paid more and buy Yelp rather than Zagat.