Maybe this is just my innate pedantry talking, but I think the opposite pathology is common too: some people are great at writing convincing but illogical/weakly-supported arguments that only fail when broken down and looked at in a granular, nitpicky way. Strong norms against that sort of response might make discussion threads less annoying on average, but they are also a boon to sophists, charismatic charlatans and sincere but overconfident bullshitters.
A nice thing about bottom-posting, and even more so inline-posting, is that is a weak certificate that the person replying has actually read the whole message.
