Thanks for the info, that's very useful.
If this is the intention, it seems like it'd make more sense to throw up roadblocks to only someone else who's in the comment chain, rather than to someone who hasn't been involved at all and is just chiming in. Usually, those flamewars are long chains of comments between two people.
It's also useful to slowdown people dog-piling the 'obvious' response to new comments.
On deeply nested comments, if it hasn't been long since the post was made you have to click the "permalink" ("XX minutes ago" or "XX hours ago") link to reply. I think the extra step is to help slow down flamewars ("do I really need to reply to this comment, right now?".