Given that it keeps comments that are replies to other comments nested under the comments that they're replying to, I suspect that that only makes a difference when one comment has two or more replies. It's much more common for a comment to have only one immediate-child-level reply, in which case it seems that the sorting doesn't make any difference in how that reply is displayed. So it's useful in some situations, but not in the majority, hence 'not very useful'.