Payrate is just one of vital components for successful campaign. I agree with payrate's contribution to quality of participants for each campaign, but it is not enough. You can pay a participant 0.005 BTC per post, but if you (as manager) or others play as managers of campaign don't have strict rules to choose participants, and after that to check their post quality before sending payment, such campaigns will end with failures and spam. Even good posters will decrease their post quality if managers are not strictly manage after choosing them into campaigns. In campaigns I joined, there are sometimes managers have to send warning message after payment sent, because some of participants show signal to turn into burst posting, that is not good for managers, and for companies.

Yobit, months ago, run their campaign with high payrates, but there is no one says that campaign is a successful one. That campaign attracted both spammers and good posters. Spammers banned temporarily if reported by users; good posters did not ban because they made posts that are good enough to not get reports from users. Because of terrible management, the number of spammers overwhelmed the number of good posters among Yobit participants.