There are many issues that get articles from multiple sources. You want us to start categorizing everything according to content instead of by url? I don't think HN is set up that way.
Actually, it's annoying to get 5 URLs pointing to different articles about the same thing in the RSS feed - if I'm interested in reading the discussion about an event, I can open up the comments for a single URL and find other interesting URLs in there.
It also means that comments about an event don't get fragmented into multiple threads.
The only reason I can see to post in separate threads is to gather karma, which is a shame as it doesn't benefit the site as a whole.
But multiple authors can have a different take on the same event. I don't want a 10-page thread with multiple links spread throughout. It's nothing to do with karma, which I could give a rat's ass about. In this case, I ignored the first story when I saw sun.com as the URL because I figured it was a bullshit press release, but read the one that said nytimes.com even though it turned out to be not much better. Over the next few months additional stories are likely to revisit the same topic with additional analysis. Lumping them all into the same thread would mean we would need to re-scan the discussion to figure out which comments we have read and which ones we haven't. By keeping the discussions fragmented we avoid that problem.
It also means that comments about an event don't get fragmented into multiple threads.
The only reason I can see to post in separate threads is to gather karma, which is a shame as it doesn't benefit the site as a whole.