You are right about there being a main idea. And then there's all the off topic-stuff that surrounds that main idea and makes the article long enough to be considered an article ;)
The value of comments are very subjective. To you, they might not be valuable, to me they might be (depending on my mood, maybe).
When talking about things, especially with people you don't know and without all the non-verbal side-info, you'll often have to agree on what you actually mean with some term you're using, and off you go from an article about any topic to a discussion of not-really-related things.
I don't have the impression that people just jump to very off topic things, but then again, you might be reading totally different submissions on HN based on your interests vs what I click on. Maybe if you gave some examples of low-value off-topic-discussions, it'd be clearer.