Its usually when there is a fundamental difference between ideas where no compromise can be made. For example, I think cities should ban personal car use while others think this is a horrible idea. We could go on and on repeating these same conversations but its not particularly useful or interesting.
Maybe, maybe not. If the goal of both people in that discussion is to convert the other to their point of view, then yes, often times these discussions are not worth having. But we're not having private conversations here. The views of both sides can educate others who are participating in the discussion or even merely observing it. Sometimes it is good to watch a discussion on a topic you feel passionate about play out without participating. While you still might get irritated by those with opposing views, it's easier to digest what they are writing/saying when it's not directed to you individually. This helps with understanding why they have a differing view and in some cases, might even help you see a weakness in your own position that you never realized was there.
