Any good campaign manager would still check the post quality, including the Merited posts. If the post isn't worth it, it shouldn't be counted as merited to qualify for the campaign.
I agree with this, there is many Legendary members who did not get not even one merit so far, and some campaigns requiring at least 5 merits, which is too low in my opinion.
Merits have become commodities, people are always looking for a way to bypass the system and if it is possible to earn on it. Buying / selling merits is just one way to get them, but there is not secret that some members use their alt accounts for merit exchange between them.
I've seen prices up to $20 per Merit. At that rate, it's even cheaper to hire someone to write a high quality post (with the risk of plagiarism!). I really don't get that people would rather pay that much for a single Merit than puting some effort in their posts.