Given, and it surely promotes their visibility when you're looking for discussion threads explicitly.

However, I find that when discussion/ask threads are mixed in with the aggregate, it's easier on the brain when they're clearly labeled as such.

Maybe I need a cognitive tune-up, but I find that I often will scan through the headlines, notice an ambiguously-titled thread, and have to do a quick judgment call: "Hm, is this a poorly-titled discussion thread or a rhetorically-titled headline?"

My argument is simply to remove those minor hiccups to improve the experience of digging through the firehose of content that comes through HN.