Yeah, in theory you post "quality" messages, you'll get rewarded.
Cannot say about others but I posted some threads which I think will benefit the forum and got some merits from different members of the forum. So, I got rewarded for quality posts in "practical."
In reality people do not send a lot of merit, slowing down by at least 100x the ranking up, for any reason - there are few merit sources here and they get a refill once a month.
Other people will judge your posts and for any reason will not send you merit even if you'll explain why the grass is green and the wind is never seen...
It won't stop in any way the spam.
Merit won't induce me to browse this forum and partecipate in any discussion I just don't want to partecipate.
And that won't stop legendary that just want to cheat, not to cheat.
If the admin feels that merit sources are not enough to cover the forum entirely, he will simple add more. Regarding senior members cheating, I agree with you. In case, such members are found, there should be some change in current system to punish them as well.