Merit system worked for a while until it didn't.
I don't know the state of things before this system, but it is clear that it does not work at all. Almost all posts everywhere I go are junk, very few are non generic garbage. If a system like this was working, over time most posts would converge on the quality of d5000's posts. As people genuinely try to do research, learn and improve their content then the average post quality must continue to rise consistently. I am all for ending the spam but let's face it, campaign managers can hire accounts with very low earned merits too if they wanted to and a person with low earned merits might be a person that produces better posts than the OG's of the forum.
While it is rare, it is possible and does occasionally happen. They probably do that individual inspections on the accounts before they hire people for signature campaigns anyway. And If they don't, that's the problem we need to solve because this problem,
Who exactly do you refer to as we? I don't see how average members could do anything about the campaign manager's behavior. The administration or the default trust people could solve it easily. actually has a real solution unlike the problem that is described in the title of this thread.
While I would not mind individual improvements such as the proposed in the OP, I agree with you. There is very bad merit farming and cycling in local sections that this would not do too much. Whenever I see an user with more merit than is warranted by his post quality, I quickly find that he has earned them in local sections for short or generic posts. 
At the beginning when the merit system was new it stopped the account farmers for a while. I'd say the forum gained a couple of spam-free years because of it but since they figured out how to game it, yes that time is over.
By "We" I mean all of us. Everybody can express his opinion here so we can all do something about it by showing our concerns.
And those who farm their merits in local boards or mega threads, they don't get to join quality high paying sig camps so easily as far as I am aware. They do join sig camps with lower pays though and that's because managers need to hire people either way and there is a shortage of good poster supply.
Let's say for a moment campaign manager refused to hire low quality legendary accounts... then they'd have to hire good posters with lower ranks, but the problem is this time, their account rank isn't high enough to display more complicated signatures.
When enough number of good quality posters emerge and get their ranks, those low quality posters with legendary accounts will be driven out of business.
Merit&account rank is only a deciding factor at the moment because the forum lacks the supply of good posters. Maybe it is not the managers' fault too as I originally thought.