Posting an affiliate link doesn’t have to mean “spamming up” Pinterest. People value the time they spend on a site differently. Some people might find more value in Pinterest if they were compensated for products they share. Fancy provides a good alternative for those people. Other sites may emerge that are more affiliate-friendly as well.

Pinterest may or may not need affiliates – the market will prove that out.

I agree that the best approach for affiliates on Pinterest is posting links back to their own sites. That said, Twitter is a good example of a user generated content site that found a balance between allowing users to post anything they want (including affiliate links) and users policing community behavior.