If the motives are pure, we'd be well-served to figure out what the other person's assumptions are in order to keep the conversation going.
Sometimes, though, we don't even know how to phrase the question of "What do you mean by x?" because we don't even know how to define x.
Often, a simple "What do you mean?" or "Can you rephrase that?" or "Can you boil down that huge freakin' wall of text?" () might actually suffice.
I'm not exactly advocating that every single person reply to every single post in a particular thread. However, if something is dismissed as irrelevant, it's often exploring why it's dismissed as relevant.
This is all, of course, assuming that most of the point of these sorts of discussions is to understand people and perspectives in the first place. In other contexts, my opinion would be different.
In almost any discussion, I tend toward really, really wanting to ensure that my perspectives end up coming across in an understandable manner and that I understand others' as well. It's definitely a priority for me. I bend toward others' mode of communication a lot. It's kind of how I'm hardwired. So, take my opinion for whatever it's worth.