"the main purpose of the merit system is to encourage those who are contributing to the forum by posting good posts." - it is just not working, it is clear forum admin fault. Spammers/bad quality posts are present in all active forums.
Who said that merit system is not working? I think the merit system is working in the best way possible. The admin implemented merit system to reduce spam on the forum and after it the number of spam posters have decreased significantly. The spam posts will always be present and they are present on every online forum of the world but in this forum merit system decreased the number of users who mostly made spam posts to increase their activity.
Can you please share proof of your claims? Where have you seen spam or bad quality posts on the forum and did those spammers earned any merits at all? If you really understand what a spam post is then you'll surely share some examples of those posts, but I'm quite sure that you won't be able to share any posts or members who earned good number of merits doing any kind of spam postings. I think you don't like the merit system because it made ranking harder for you and all those members who have less knowledge about Bitcoin and can't contribute good stuff to the forum, but those who have knowledge about Bitcoin and can contribute something good to the forum will always have more than enough merit to rank up.
In fact, I have seen some members who ranked up pretty fast and have more than enough merits for their next rank. Those members are contributing good posts to the forum and that's why they are getting those merits. If you can do the same then trust me the members will merit you for your contribution. You have got more than 15 merits for a single post that someone considered helpful and that proves the merits are always there for good and helpful posts.