Few reasons why we still have some high-quality posters without signature offers; they have influence, but their posting habits will not benefit the manager or client--they are good, but not everyone deserves to be paid.
That's exactly what I'm talking about. If you don't like someone's posting habits, you simply don't invite them to your campaign. And if you fill the campaign with only those whose posting habits fits you, you shouldn't need a minimum post requirement as they will write required amount of posts anyway.
Having said that, I do understand that it's easier said than done, as realistically there are more spots in campaigns than there are decent posters. As a result, managers often have to make compromises and hire below average posters, imposing bunch of restrictions and rules in order to get some results.