Is it ever okay to be mean to the person you're arguing with? If so, under what circumstances and why?

Some people are almost always nice. Even while they're arguing, they'll act as nice and polite as possible. Before and after the criticisms, they'll insert some friendly points of agreement and make sure the opponents know they're still respected - no matter how stupid what they're saying is. This makes sense in a way. If people like you more, they'll feel less threatened, they'll be more likely to continue talking and might even be more willing to change their views.

On the other hand, I've heard the argument that some people need to be jolted out of their set beliefs with a bit of unfriendly criticism. When their beliefs are criticized too lightly and are decorated with compliments and points of agreement, they aren't as likely to seriously reconsider their beliefs. Instead, they'll dwell on the positive things and leave the arguments thinking they made new friends, that "I believe this and she believes that and we can accept our differences." Niceness only lightens the issues.

Furthermore, it's often easier to argue with a bit of maliciousness. It's sometimes easier to explain why someone is wrong by using some cheeky, mean joke, a metaphor or analogy, or simply by being blunt. For example, if you think the person is being overall irrational, is simply not using logic correctly, there is really no way to tell them that without sounding mean. So if someone is intent on remaining nice, things like that will remain unsaid.