Discussions tend to spill over across threads. If there's a population that tends to discuss politics, they're not necessarily going to keep the political discussions in the political threads, nor should they be expected to. And people have different definitions of what constitutes political.

And if this lends itself to allowing more heated debate (because the political threads are expected to be isolated in some way), that same heated debate form will likewise spill over into other threads.

But what hiding stories does accomplish is preventing people who don't want to see that content from being angered by its presence, especially when a topic threatens to overwhelm the site, and otherwise takes up most of the comments feed in a heated discussion, or most of the new topics page in the case of a popular story.

Although a more proactive approach would be to better curate the comments feed. It still seems that high traffic or volatile threads contribute a lot of low quality spillover there, and a lot of low hanging fruit for flamebaiting and drive-by comments.

True. It doesn't stop it. I think it does limit it, to some extent.

a more proactive approach would be to better curate the comments feed.

Interesting. Do you think the community would bear more aggressive curation?

As it wouldn't affect the rank of the stories themselves, I don't see why not. It might not need human intervention either, just apply a few rules before building the comment page such as not displaying comments from a certain depth (which tend to be tangential) or not displaying downvoted comments, and preferring upvoted comments.