I wouldn't think so, just because the replies are good and worthy of a ++, doesn't always mean that the root node is. It could be someone posted a really insightful reply to a not-so-insightful question, or (as is often the case here on PM) the reply may not even have anything to do with the root node at all! Also, this would change the structure of XP gain quite a bit which, as far as I can tell, is generally discouraged as most people are fairly happy with the way things are now and changing something that big would only start a big debate.