Silvery - 7:07 pm on Aug 24, 2010 (gmt 0)
From an SEO perspective, Google seems to place a lot of weight in sites which allow/provide reviews of businesses. It appears to be one of a number of features which enable a webmaster to differentiate their content sufficiently to achieve some good rankings -- for business directory sites, merely displaying Biz Name, Address, City, State and Phone are not all that sufficient in Google's eyes to allow good rankings of the page -- multiple online directories showing the same basic listing info could engage duplicate content filters in search engines.
So, enabling reviews seems to help with this.
It's possible that your concerns might be out of whack with the actual risks. Particularly, if you try to build a reviews system/policy which provides some level of balance. Good policing of obvious review manipulation attempts, allowing businesses to respond in some way, etc.
There's definitely some need to police reviews coming in, but it's possible to do on a low-cost basis algorithmically, and perhaps in combo with some lite human interventions.
These practices could reduce liability and expense.