[s1artibartfast]

I think the main problem raised by your statement is that people often don't have a sense of why they are talking or what the conversation is about. This is especially true on the internet, or as far as I can tell people are often just posting the first thing that comes into their mind. Speaking stipulatively may or may not be relevant, and that relevance depends on the subject and purpose at hand

[dataflow]

Or the issue is you can't find a polite way to communicate it at that point. Like in the flat earth case, "your premise is so insane that I don't want to waste another second of my life debating it with you" would... not go over well - neither with the second party, nor with the moderators of the forum facilitating it.