I wouldn't think too many as the arrow is pointing up and not down as it would be for expanded comments. In addition, it exists on every question/answer regardless if there are responses.
But I see your point that the context is subtle and maybe some people would miss it and still click.
Just my opinion too, but HN is a uber simplistic interface that isn't following a lot of current trends, but it also isn't really designed for the consumer level user either.
