I agree this is not the best model or maybe better to say most fair for users. If anyone is forced to post xx times per week to get pay, then there is no choice but to do it, even though it results in spam sometimes.
Here is my thinking, and you can feel free to dig in as deep as you want to reject my logic.
If you are a strict campaign manager that seeks out only high-quality posts, but accepts a range of participants with varying post qualities, then the advertiser will benefit from the posts that are not eligible for payment.
For example: suppose we have a pay/post campaign in which there are 10 users with a cap of 25 posts/week. They post 200 posts in total over the span of the round. Of those 200, roughly 120 are eligible for payment.
What then happens is that you have higher quality posts than a fixed-rate campaign of 10 x 20 posts. Not only that, but you have the ability to reward the high-quality posters if the pay/post totals exceed that of the fixed rate.
The other part is that campaign participants are still rewarded for the 12 eligible posts they created rather than being subject to a loss of the week's funds just because they fell under the target. What you might have with fixed-rate campaigns is "post padding" in which they will forcibly post more than the amount to secure their payment, which is not the kind of behavior that induces (generally) good posts.
I see few drawbacks, except in the case where all the campaign participants that are accepted seldom post, to which the advertiser suffers if they want short-term gains. However, over the long-term, they will actually see
more traffic with the same amount of funds. The best model for garnering long-term attention would be a medium-to-high reward pay/post campaign with a relatively low cap on the maximum rewarded posts for each round.