On the SC2 forums, it means a lot. An opinion coming from a Master player means much more then from someone in Bronze league. Also, I just think it looks cool to display your achievements along with each post.
They word you are looking for is Logical Fallacy. The poster posting an argument has nothing to do with the merit of the argument.
The background knowledge of the poster means a lot. It's much the same way as trying to argue with your doctor when he's told you that you need to lose weight or you will die from a heart attack. On the SC2 forums, there's a fine line between trying to show a lower league Bronze player that he is wrong while you are a higher level Master player. Any mention of his league can lead to an auto warning. You get 3 of those warnings and you have a permanent ban.