In some respects, getting a "vicious" person can be easier to deal with, since you should have an easier time tearing apart their arguments (assuming the moderator is remotely competent at granting a chance to respond).
Having an argument with "two sides" is not always representative of the population or the quality of ideas, and fringe/activist elements tend to fight hardest for those seats. Sometimes you get someone very thoughtful, and sometimes you get garbage.