Brands fear a type of riot on their sites led by a few hardliners who will infect the waverers and the positive visitors with their negative opinions resulting in everyone becoming more extreme. The tendency to advocate more extreme positions in a group is called group polarisation. Social Psychologists, Moscovici and Zavalloni, coined the term, and is related to the less rational phenomena of groupthink. So with group polarisation the 'slightly sexist' become more sexist , and 'marginally racist' become virulently racist after participation in a group discussion. Studies have shown that group polarisation is even more extreme online than offline. So some of the nervousness my clients have shown is warranted.
But brands don't have to surrender control to anarchy if they open up to community discussion. Martin Luther King guided a group, and turned it into a movement. A movement that was angry, but at the same time pacifist, a movement looking for change, but driven by age-long ideals. A positive form of group polarisation is possible if it's driven by leadership that shapes the debate, sets the agenda, and moderates the discussion by providing a balanced counterpoint to the opposition's arguments. This also means being open and honest. Show both sides of the argument, compartmentalise the debate into different discussion streams - which you define. Start the community on topics peripheral to the brand's core theme, then gradually move towards the core. Use advocates of the brand to be your evangelists, and get them to share their experiences with people in their social graphs. And perhaps most importantly, don't see it as a zero-sum game with a winner and a loser, but instead a mechanism for funneling the collective intelligence of your stakeholders into your brand. There's wisdom in those crowds.