May I ask you this question: Who does or do the quality post? Is the post not posted by the quality poster?. You award merit to a quality post which was done by a quality poster and the poster takes the Glory and not the post.
Anyone can post quality posts not just those "quality posters" and even those quality posters can sometime post a non-quality posts. What I'm trying to say is not all posts of a quality posters are good quality and deserves Merits. Anyone can make good posts.
Those "Quality Posters" that you were talking about are those members who often contribute in some discussions and often received Merits and they deserve Merits as they were giving efforts to share knowledge in discussions.
And anyone can be a quality poster.