I agree with you, the quality of post is subjective because everyone has a different way of judging. About quality post not getting merit in my opinion because this merit is not moderated so the user can still ignore those post from merit. The solution may already exist, please report the post on the merit sources in a specially created thread.

Quality post are part of the contribution to forum and users. The quality of user post has a positive impact on the forum as it help the forum reduce the number of junk post. In particular, this merit system is assumed to force users to post quality things so that the benefit will guarantee them the merit that play an important role in ranking up. Contribution is sometime not always about creating a new thread, it could also be because someone want to help solve some problem that the user is experiencing.