Merit system is introduced to nullified low quality posts in the forum. but something is going exactly opposite to this.
I don't know how you came to this conclusion, but I'm going to have to completely disagree. The merit system is not meant to do anything to low-quality posts; the merit system is a form of positive reinforcement, because it rewards good behavior, it does not punish bad behavior. Therefore, the merit system is meant to reward quality posts. You say the opposite is happening, so you believe that the merit system is encouraging low-quality posts? I'm not sure how you can say this confidently, there is no evidence for this.
I am seeing more rubbish posts, as everyone is trying to make more and more posts which shows the insecurity among them.
You're saying people are spamming more, as a result of the merit system. Do you have any statistics or information to support this theory or are you just talking out of your ass? I would hate to think you are speaking only off of an anecdote.
Do merit system really make a difference in this forum?
Yes. I have seen many spammers run-off crying from the forum already and it has given me new ambition to actually come hangout again. The merit system makes it so ranking up is extremely limited, as it should be, especially when it comes with any sort of perks.