The off-topic talk is necessary to make connections and to make everyone on a forum feel more at home. It is also a way to get to know eachother a little bit better. This sometimes lead to people realizing that they don't share the same viewpoint on different things.
This is something that some people have hard to accept, and sometimes it even bugs people to the extreme that they either leave or tries to make the other person to leave.
That is why we sometimes see topics evolving into pure flame wars and that is when the moderators step in and take appropriate actions.
Sometimes the mods actions can be questionable, but this should be delt with internally between the mod and the instigators.
If two people realize that they can't stand eachother, this should also be delt with personally via pm or email and not as a topic on a forum.
...just my two cents