I think the idea is that if people don't see new replies to the hidden subthread in recent comments, they'll be much less likely to respond to those replies, so such threads will die out much more quickly. This will also cause trolls to not have as much fun trolling here so they'll be more likely to leave us alone in the future.

ETA: On the other hand, perhaps we should talk about non-technical ways to change the culture as well. Do you have any ideas? ETA2: A lot of previous discussion can be found here.