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thank you
It is not their job to increase your site's placement in the search engine listings, and in fact is counterproductive for them as they are effectively helping you to compete with them.
I would suggest you focus on creating a cooperative relationship with your affiliates rather than a competitive one.
If you want to increase your site's PR, do it the old fashioned way, by hunting out quality links, and forget about using your affiliates' links for this purpose.